Add Git Branching Workshop - exercises to build branching mental model

common-content/en/blocks/git-branching-workshop/index.md

@@ -0,0 +1,175 @@
++++
+title = 'Git Branching Workshop'
+time = 60
+[tasks]
+    1='Explain the difference between local branches and remote branches'
+    2='Create a new branch for a new piece of coursework'
+    3='Submit a PR that contains only the commits for that piece of work'
+    4='Identify when a PR contains commits from the wrong branch'
+[objectives]
+    1='Create a branch from main and keep it separate from other work'
+    2='Submit a clean PR with only the relevant commits'
+    3='Diagnose a mixed-commit PR and fix it'
+[build]
+  render = 'never'
+  list = 'local'
+  publishResources = false
++++
+
+This workshop addresses a problem we see regularly: trainees submit PRs that contain commits from other pieces of coursework. This happens because of a misconception about how branches work. This workshop builds the right mental model.
+
+Before this workshop, make sure you have completed [Using GitHub](../using-github) and read the [Trainee PR Guide](../../guides/reviewing/trainee-pr-guide).
+
+Also look at the [branching notes in the Onboarding module](https://curriculum.codeyourfuture.io/itp/onboarding/sprints/1/prep/#branching) for a visual guide to creating branches.
+
+## The Core Problem
+
+When you submit coursework, each piece should come from its own branch. If you make two pieces of coursework on the same branch, your PR for coursework B will also contain all the commits from coursework A. This makes it impossible to review and merge them separately.
+
+This workshop will show you:
+1. Why this happens
+2. How to prevent it
+3. How to diagnose it if it goes wrong
+
+---
+
+## Part 1: Understanding Branches Visually (15 minutes)
+
+### Exercise: Map the Branch Structure
+
+Look at these two diagrams of a repository.
+
+**Diagram 1: Clean branch structure**
+
+```mermaid
+gitGraph
+   commit id: "A"
+   commit id: "B"
+   commit id: "C"
+   commit id: "D"
+   commit id: "F"
+   branch topic-1
+   checkout topic-1
+   commit id: "E"
+```
+
+**Diagram 2: Problem - topic-2 was branched from topic-1, not main**
+
+```mermaid
+gitGraph
+   commit id: "A"
+   commit id: "B"
+   commit id: "C"
+   commit id: "D"
+   commit id: "F"
+   branch topic-1
+   checkout topic-1
+   commit id: "E"
+   commit id: "G"
+   commit id: "H"
+   branch topic-2
+   checkout topic-2
+   commit id: "I"
+   commit id: "J"
+```
+
+**Task:** In your own words, answer these questions:
+
+1. In Diagram 1, which commits belong to topic-1?
+2. In Diagram 2, what went wrong with topic-2?
+3. If you submitted a PR for topic-2 from Diagram 2, what would it contain?
+4. What should topic-2 have been branched from?
+
+Share your answers with your group.
+
+---
+
+## Part 2: Create Two Branches Correctly (25 minutes)
+
+### Exercise: Two Coursework Pieces, Two Branches
+
+Your trainer will share a link to a practice repository. For this exercise, you will complete two separate pieces of coursework on two separate branches.
+
+**Important:** Each branch must be created from `main`, not from the other branch.
+
+These steps use the terminal. If you prefer, you can do this in VSCode using the Source Control panel instead -- the concepts are the same, only the interface is different.
+
+**Step 1: Branch for Coursework 1**
+
+- Clone the repository
+- Create a new branch from `main` called `cw1`
+- Make a change (edit any file), commit it
+- Push `cw1` to your fork
+
+**Step 2: Verify your branch is clean**
+
+Before you make any more commits, check what commits are on your branch. In your terminal, run:
+
+```bash
+git log main..HEAD
+```
+
+This shows commits on your branch that are not on main. If it shows more than the commits you just made, something is wrong.
+
+**Step 3: Create Branch for Coursework 2**
+
+Now create a second branch, also from `main`. In your terminal:
+
+```bash
+git checkout main
+git pull origin main
+git checkout -b cw2
+```
+
+Make a different change, commit it, push `cw2`.
+
+**Step 4: Verify cw2 is separate**
+
+Check the log for cw2. In your terminal:
+
+```bash
+git log main..HEAD
+```
+
+How many commits does it show? They should only be the one you just made.
+
+**What went wrong in the bad example:** The trainee created `cw2` from `cw1` instead of from `main`. This means `cw2` contains all of `cw1`'s commits as well.
+
+---
+
+## Part 3: Diagnosing a Mixed-Commit PR (20 minutes)
+
+### Exercise: Find the Problem
+
+Your trainer will share a link to a PR that has a problem. This PR contains commits that should not be there.
+
+Use the GitHub interface to answer these questions:
+
+1. How many commits does this PR contain?
+2. Look at the commit list. Are there any commits that do not belong to this piece of work?
+3. Can you identify which branch these extra commits came from?
+4. How would you fix this?
+
+**Hints:**
+
+- Click on the commits tab to see the full list
+- Click on individual commits to see what changed
+- Look for commits with messages that do not match the coursework description
+
+### Discussion (5 minutes)
+
+As a group, discuss:
+
+- What was the problem in this PR?
+- How would you avoid making the same mistake?
+- What command would you run locally to check your PR before submitting?
+
+---
+
+## Key Takeaways
+
+- **Every piece of coursework gets its own branch, branched from `main`**
+- If you need to start new coursework while waiting for a review, always branch from `main`, not from your current work
+- `git log main..HEAD` tells you exactly what commits are on your branch and not on `main`
+- Before submitting a PR, check the commits tab. Does every commit belong to this piece of work?
+- If your PR has extra commits, the fix is: create a new branch from `main`, cherry-pick only the right commits, submit a new PR