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