Cover submodule working trees in safe.directory on Cygwin

Add the gitdb and smmap submodule working tree paths to the
`safe.directory` config in the Cygwin CI workflow. Without this,
when GitPython opens a submodule as a `Repo` and runs
`git cat-file --batch-check` against it, git rejects the repository
for dubious ownership. The user-visible failure modes documented in
commit 1 (`ValueError`, `IndexError`, `AssertionError`) all trace
back to this rejection.

Why `[gitdb]` fails and `[smmap]` incidentally passes
-----------------------------------------------------

The trust check in `test/test_fixture_health.py` fails for `[gitdb]`
but passes for `[smmap]` even though neither submodule's working
tree is in the pre-fix `safe.directory` list. The asymmetry comes
down to which Owner SID NTFS records on the gitdb working tree
directory.

`actions/checkout`'s `git clone` materializes the main repo's tree
using the Windows git binary. The tree contains a submodule entry
at `git/ext/gitdb` (mode 160000), which `git checkout` realizes as
an empty directory at that path. On the GHA windows-latest runner
with UAC effectively disabled, the runneradmin process holds the
full admin token, and Windows stamps the new directory's NTFS Owner
as `BUILTIN\Administrators` (Cygwin uid 544) rather than the
runneradmin individual user (Cygwin uid 197108). The same is true
of the surrounding workspace directories (`git`, `git/ext`, `.git`,
etc.) that `actions/checkout` also creates.

When `init-tests-after-clone.sh` later runs Cygwin's
`git submodule update --init --recursive`:

- Cygwin git clones gitdb's contents into the existing
  `git/ext/gitdb` placeholder. The directory's top-level Owner stays
  `BUILTIN\Administrators`. Files Cygwin creates inside it (the
  `.git` gitlink, gitdb's contents) are owned by runneradmin.
- Cygwin git creates `.git/modules/gitdb` (gitdb's gitdir) from
  scratch; runneradmin-owned.
- For smmap, Cygwin git creates the entire chain
  `git/ext/gitdb/gitdb/ext/smmap` (worktree) and
  `.git/modules/gitdb/modules/smmap` (gitdir) from scratch, since
  none of those paths existed pre-update; all runneradmin-owned.

Cygwin's `lstat().st_uid` reports the actual NTFS Owner SID mapped
to a POSIX uid. Git's `is_path_owned_by_current_user` reduces to
`lstat(p).st_uid == geteuid()` on Cygwin (no special-casing), and
`ensure_valid_ownership` fast-paths the repository when ALL of
(gitfile, worktree, gitdir) pass that check; otherwise it falls
through to matching `safe.directory` against the worktree's
`real_pathdup`.

For gitdb: gitfile=runneradmin, **worktree=Administrators**,
gitdir=runneradmin. The fast-path AND fails on the worktree.
`safe.directory` contains only `$(pwd)` and `$(pwd)/.git` pre-fix,
neither of which exact-matches `git/ext/gitdb`. Repository rejected.

For smmap: all three paths runneradmin-owned. Fast-path accepts.

Cygwin's `chown` cannot rewrite the gitdb worktree's Owner SID from
Administrators to runneradmin (it returns "Permission denied" even
with admin token). Adding both submodule worktree paths to
`safe.directory` is the correct fix, and is robust against future
changes in which paths happen to inherit which Owner.

Why `actions/checkout`'s own `safe.directory` doesn't help
---------------------------------------------------------

`actions/checkout`'s `set-safe-directory: true` default adds the
main repository path to `safe.directory`, but on Cygwin that
addition lands in the Windows-side global git config (under
runneradmin's Windows HOME), not the Cygwin-side one (under the
Cygwin user's HOME). Cygwin git reads only its own global config,
so `actions/checkout`'s entry is invisible to it. The cygwin-test
workflow therefore explicitly populates Cygwin git's
`safe.directory` itself; this commit extends that to cover the
gitdb and smmap working trees.

Verification
------------

The `reproduce-safe-dir` matrix on the previous commit produces
failures for all three affected tests; this commit's CI run is
expected to show those tests passing instead.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: EliahKagan

.github/workflows/cygwin-test.yml

@@ -58,6 +58,8 @@ jobs:
       run: |
         git config --global --add safe.directory "$(pwd)"
         git config --global --add safe.directory "$(pwd)/.git"
+        git config --global --add safe.directory "$(pwd)/git/ext/gitdb"
+        git config --global --add safe.directory "$(pwd)/git/ext/gitdb/gitdb/ext/smmap"
         git config --global core.autocrlf false
 
     - &prepare-repo