ci: install supported Git in the normal Cygwin layout

The Cygwin full suite reached 1,372 passing tests but failed its native-Git
detection check. The new source build installed Git into `/usr/local`,
while GitPython's existing detector expects `uname` beside the selected
Git executable. Cygwin installs `uname` in its normal `/usr/bin` directory.

Install Git 2.52 under `/usr`, replacing the older packaged Git and
preserving that standard layout. Verify `Git.is_cygwin()` immediately
after installing Python dependencies so a setup regression fails before
the long test suite. The detector and its missing-`uname` behavior remain
unchanged.

YAML parsing, extracted Bash syntax, ShellCheck, and `git diff --check`
pass locally. The preceding Cygwin performance suite also passed all six
tests; native validation of the corrected installation runs in CI.

Author: codex

.github/workflows/cygwin-test.yml

@@ -59,7 +59,8 @@ jobs:
       run: |
         # Cygwin's packaged Git is older than our minimum supported version.
         git clone --depth 1 --branch v2.52.0 -- https://github.com/git/git.git /tmp/git-source
-        make -C /tmp/git-source -j2 prefix=/usr/local NO_GETTEXT=YesPlease NO_TCLTK=YesPlease NO_PERL=YesPlease install
+        # Use Cygwin's normal prefix: GitPython checks for uname next to git.
+        make -C /tmp/git-source -j2 prefix=/usr NO_GETTEXT=YesPlease NO_TCLTK=YesPlease NO_PERL=YesPlease install
         hash -r
         test "$(git version)" = 'git version 2.52.0'
 
@@ -97,6 +98,9 @@ jobs:
       run: |
         pip install ./smmap ./gitdb '.[test]'
 
+    - name: Verify Cygwin Git detection
+      run: python -c 'from git import Git; assert Git.is_cygwin()'
+
     - name: Show POSIX file ownership
       # Cygwin's `ls -ld` reports the NTFS Owner SID via Cygwin's SID-to-uid
       # mapping (well-known SIDs by their RID, machine-local accounts by