doc: add a explanation of Git's data model

Git very often uses the terms "object", "reference", or "index" in its
documentation.

However, it's hard to find a clear explanation of these terms and how
they relate to each other in the documentation. The closest candidates
currently are:

1. `gitglossary`. This makes a good effort, but it's an alphabetically
    ordered dictionary and a dictionary is not a good way to learn
    concepts. You have to jump around too much and it's not possible to
    present the concepts in the order that they should be explained.
2. `gitcore-tutorial`. This explains how to use the "core" Git commands.
   This is a nice document to have, but it's not necessary to learn how
   `update-index` works to understand Git's data model, and we should
   not be requiring users to learn how to use the "plumbing" commands
   if they want to learn what the term "index" or "object" means.
3. `gitrepository-layout`. This is a great resource, but it includes a
   lot of information about configuration and internal implementation
   details which are not related to the data model. It also does
   not explain how commits work.

The result of this is that Git users (even users who have been using
Git for 15+ years) struggle to read the documentation because they don't
know what the core terms mean, and it's not possible to add links
to help them learn more.

Add an explanation of Git's data model. Some choices I've made in
deciding what "core data model" means:

1. Omit pseudorefs like `FETCH_HEAD`, because it's not clear to me
   if those are intended to be user facing or if they're more like
   internal implementation details.
2. Don't talk about submodules other than by mentioning how they
   relate to trees. This is because Git has a lot of special features,
   and explaining how they all work exhaustively could quickly go
   down a rabbit hole which would make this document less useful for
   understanding Git's core behaviour.
3. Don't discuss the structure of a commit message
   (first line, trailers etc).
4. Don't mention configuration.
5. Don't mention the `.git` directory, to avoid getting too much into
   implementation details

Signed-off-by: Julia Evans <julia@jvns.ca>

Author: jvns

Documentation/Makefile

@@ -52,6 +52,7 @@ MAN7_TXT += gitcli.adoc
 MAN7_TXT += gitcore-tutorial.adoc
 MAN7_TXT += gitcredentials.adoc
 MAN7_TXT += gitcvs-migration.adoc
+MAN7_TXT += gitdatamodel.adoc
 MAN7_TXT += gitdiffcore.adoc
 MAN7_TXT += giteveryday.adoc
 MAN7_TXT += gitfaq.adoc

Documentation/gitdatamodel.adoc

@@ -0,0 +1,281 @@
+gitdatamodel(7)
+===============
+
+NAME
+----
+gitdatamodel - Git's core data model
+
+SYNOPSIS
+--------
+gitdatamodel
+
+DESCRIPTION
+-----------
+
+It's not necessary to understand Git's data model to use Git, but it's
+very helpful when reading Git's documentation so that you know what it
+means when the documentation says "object", "reference" or "index".
+
+Git's core operations use 4 kinds of data:
+
+1. <<objects,Objects>>: commits, trees, blobs, and tag objects
+2. <<references,References>>: branches, tags,
+   remote-tracking branches, etc
+3. <<index,The index>>, also known as the staging area
+4. <<reflogs,Reflogs>>: logs of changes to references ("ref log")
+
+[[objects]]
+OBJECTS
+-------
+
+Commits, trees, blobs, and tag objects are all stored in Git's object database.
+Every object has:
+
+[[object-id]]
+1. an *ID* (aka "object name"), which is a cryptographic hash of its
+  type and contents.
+  It's fast to look up a Git object using its ID.
+  This is usually represented in hexadecimal, like
+  `1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a`.
+2. a *type*. There are 4 types of objects:
+   <<commit,commits>>, <<tree,trees>>, <<blob,blobs>>,
+   and <<tag-object,tag objects>>.
+3. *contents*. The structure of the contents depends on the type.
+
+Once an object is created, it can never be changed.
+Here are the 4 types of objects:
+
+[[commit]]
+commits::
+    A commit contains these required fields
+    (though there are other optional fields):
++
+1. All the *files* in the commit, stored as the *<<tree,tree>>* ID of
+   the commit's base directory.
+2. Its *parent commit ID(s)*. The first commit in a repository has 0 parents,
+  regular commits have 1 parent, merge commits have 2 or more parents
+3. An *author* and the time the commit was authored
+4. A *committer* and the time the commit was committed.
+   If you cherry-pick (linkgit:git-cherry-pick[1]) someone else's commit,
+   then they will be the author and you'll be the committer.
+5. A *commit message*
++
+Here's how an example commit is stored:
++
+----
+tree 1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a
+parent 4ccb6d7b8869a86aae2e84c56523f8705b50c647
+author Maya <maya@example.com> 1759173425 -0400
+committer Maya <maya@example.com> 1759173425 -0400
+
+Add README
+----
++
+Like all other objects, commits can never be changed after they're created.
+For example, "amending" a commit with `git commit --amend` creates a new
+commit with the same parent.
++
+Git does not store the diff for a commit: when you ask Git for a
+diff it calculates it on the fly.
+
+[[tree]]
+trees::
+    A tree is how Git represents a directory. It lists, for each item in
+    the tree:
++
+[[file-mode]]
+1. The *file mode*, for example `100644`. The format is inspired by Unix
+   permissions, but Git's modes are much more limited. Git only supports these file modes:
++
+  - `100644`: regular file (with type `blob`)
+  - `100755`: executable file (with type `blob`)
+  - `120000`: symbolic link (with type `blob`)
+  - `040000`: directory (with type `tree`)
+  - `160000`: gitlink, for use with submodules (with type `commit`)
+
+2. The *type*: either <<blob,`blob`>> (a file), `tree` (a directory),
+  or <<commit,`commit`>> (a Git submodule, which is a
+  commit from a different Git repository)
+3. The <<object-id,*object ID*>>
+4. The *filename*
++
+For example, this is how a tree containing one directory (`src`) and one file
+(`README.md`) is stored:
++
+----
+100644 blob 8728a858d9d21a8c78488c8b4e70e531b659141f README.md
+040000 tree 89b1d2e0495f66d6929f4ff76ff1bb07fc41947d src
+----
+
+
+[[blob]]
+blobs::
+    A blob is how Git represents a file. A blob object contains the
+    file's contents.
++
+When you make a new commit, Git only needs to store new versions of
+files which were changed in that commit. This means that commits
+can use relatively little disk space even in a very large repository.
+
+[[tag-object]]
+tag objects::
+    Tag objects contain these required fields
+    (though there are other optional fields):
++
+1. The *ID* and *type* of the object (often a commit) that they reference
+2. The *tagger* and tag date
+3. A *tag message*, similar to a commit message
+
+Here's how an example tag object is stored:
+
+----
+object 750b4ead9c87ceb3ddb7a390e6c7074521797fb3
+type commit
+tag v1.0.0
+tagger Maya <maya@example.com> 1759927359 -0400
+
+Release version 1.0.0
+----
+
+NOTE: All of the examples in this section were generated with
+`git cat-file -p <object-id>`, which shows the contents of a Git object.
+
+[[references]]
+REFERENCES
+----------
+
+References are a way to give a name to a commit.
+It's easier to remember "the changes I'm working on are on the `turtle`
+branch" than "the changes are in commit bb69721404348e".
+Git often uses "ref" as shorthand for "reference".
+
+References can either be:
+
+1. References to an object ID, usually a <<commit,commit>> ID
+2. References to another reference. This is called a "symbolic reference".
+
+References are stored in a hierarchy, and Git handles references
+differently based on where they are in the hierarchy.
+Most references are under `refs/`. Here are the main types:
+
+[[branch]]
+branches: `refs/heads/<name>`::
+    A branch is a name for a commit ID.
+    That commit is the latest commit on the branch.
++
+To get the history of commits on a branch, Git will start at the commit
+ID the branch references, and then look at the commit's parent(s),
+the parent's parent, etc.
+
+[[tag]]
+tags: `refs/tags/<name>`::
+    A tag is a name for a commit ID, tag object ID, or other object ID.
+    Tags that reference a tag object ID are called "annotated tags",
+    because the tag object contains a tag message.
+    Tags that reference a commit, blob, or tree ID are
+    called "lightweight tags".
++
+Even though branches and tags are both "a name for a commit ID", Git
+treats them very differently.
+Branches are expected to change over time: when you make a commit, Git
+will update your <<HEAD,current branch>> to reference the new changes.
+Tags are usually not changed after they're created.
+
+[[HEAD]]
+HEAD: `HEAD`::
+    `HEAD` is where Git stores your current <<branch,branch>>.
+    `HEAD` can either be:
+    1. A symbolic reference to your current branch, for example `ref:
+       refs/heads/main` if your current branch is `main`.
+    2. A direct reference to a commit ID. This is called "detached HEAD
+	   state", see the DETACHED HEAD section of linkgit:git-checkout[1] for more.
+
+[[remote-tracking-branch]]
+remote tracking branches: `refs/remotes/<remote>/<branch>`::
+    A remote-tracking branch is a name for a commit ID.
+    It's how Git stores the last-known state of a branch in a remote
+    repository. `git fetch` updates remote-tracking branches. When
+    `git status` says "you're up to date with origin/main", it's looking at
+    this.
++
+`refs/remotes/<remote>/HEAD` is a symbolic reference to the remote's
+default branch. This is the branch that `git clone` checks out by default.
+
+[[other-refs]]
+Other references::
+    Git tools may create references anywhere under `refs/`.
+    For example, linkgit:git-stash[1], linkgit:git-bisect[1],
+    and linkgit:git-notes[1] all create their own references
+    in `refs/stash`, `refs/bisect`, etc.
+    Third-party Git tools may also create their own references.
++
+Git may also create references other than `HEAD` at the base of the
+hierarchy, like `ORIG_HEAD`.
++
+NOTE: By default, Git references are stored as files in the `.git` directory.
+For example, the branch `main` is stored in `.git/refs/heads/main`.
+This means that you can't have branches named both `maya` and `maya/some-task`,
+because there can't be a file and a directory with the same name.
+
+[[index]]
+THE INDEX
+---------
+
+The index, also known as the "staging area", contains a list of every
+file in the repository and its contents. When you commit, the files in
+the index are used as the files in the next commit.
+
+You can add files to the index or update the version in the index with
+linkgit:git-add[1]. Adding a file to the index or updating its version
+is called "staging" the file for commit.
+
+Unlike a <<tree,tree>>, the index is a flat list of files.
+Each index entry has 4 fields:
+
+1. The *<<file-mode,file mode>>*
+2. The *<<blob,blob>> ID* of the file
+3. The *file path*, for example `src/hello.py`
+4. The *stage number*, either 0, 1, 2, or 3. This is normally 0, but if
+   there's a merge conflict there can be multiple versions of the same
+   filename in the index.
+
+It's extremely uncommon to look at the index directly: normally you'd
+run `git status` to see a list of changes between the index and <<HEAD,HEAD>>.
+But you can use `git ls-files --stage` to see the index.
+Here's the output of `git ls-files --stage` in a repository with 2 files:
+
+----
+100644 8728a858d9d21a8c78488c8b4e70e531b659141f 0 README.md
+100644 665c637a360874ce43bf74018768a96d2d4d219a 0 src/hello.py
+----
+
+[[reflogs]]
+REFLOGS
+-------
+
+Every time a branch, remote-tracking branch, or HEAD is updated, Git
+updates a log called a "reflog" for that <<reference,reference>>.
+This means that if you make a mistake and "lose" a commit, you can
+generally recover the commit ID by running `git reflog <reference>`.
+
+Each reflog entry has:
+
+1. Before/after *commit IDs*
+2. *User* who made the change, for example `Maya <maya@example.com>`
+3. *Timestamp* when the change was made
+4. *Log message*, for example `pull: Fast-forward`
+
+Reflogs only log changes made in your local repository.
+They are not shared with remotes.
+
+For example, here's how the reflog for `HEAD` in a repository with 2
+commits is stored:
+
+----
+0000000000000000000000000000000000000000 4ccb6d7b8869a86aae2e84c56523f8705b50c647 Maya <maya@example.com> 1759173408 -0400      commit (initial): Initial commit
+4ccb6d7b8869a86aae2e84c56523f8705b50c647 750b4ead9c87ceb3ddb7a390e6c7074521797fb3 Maya <maya@example.com> 1759173425 -0400      commit: Add README
+----
+
+GIT
+---
+Part of the linkgit:git[1] suite

Documentation/glossary-content.adoc

@@ -297,8 +297,8 @@ This commit is referred to as a "merge commit", or sometimes just a
 	identified by its <<def_object_name,object name>>. The objects usually
 	live in `$GIT_DIR/objects/`.
 
-[[def_object_identifier]]object identifier (oid)::
-	Synonym for <<def_object_name,object name>>.
+[[def_object_identifier]]object identifier, object ID, oid::
+	Synonyms for <<def_object_name,object name>>.
 
 [[def_object_name]]object name::
 	The unique identifier of an <<def_object,object>>.  The

Documentation/meson.build

@@ -192,6 +192,7 @@ manpages = {
   'gitcore-tutorial.adoc' : 7,
   'gitcredentials.adoc' : 7,
   'gitcvs-migration.adoc' : 7,
+  'gitdatamodel.adoc' : 7,
   'gitdiffcore.adoc' : 7,
   'giteveryday.adoc' : 7,
   'gitfaq.adoc' : 7,