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
the previous commit (`ValueError`, `IndexError`, `AssertionError`)
all trace back to this rejection.

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