ci: build supported Git for Cygwin tests

The Cygwin performance job failed all six setup phases because the Cygwin
package currently supplies Git 2.51.0, below the new Git 2.52 minimum.
The same mismatch prevents the regular Cygwin suite from opening repositories.

Build upstream Git `v2.52.0` with Cygwin tools and install it in `/usr/local`
before preparing the fixtures. Include the HTTPS development dependencies
so the existing clone and remote tests retain network transport support.
Verify that the selected `git` is the built version before continuing.

Validation: the workflow parses as YAML, the added shell block passes
`bash -n` and ShellCheck, and `git diff --check` passes. The native Cygwin
build and test suites require the Windows CI runner.

Author: codex

.github/workflows/cygwin-test.yml

@@ -45,14 +45,24 @@ jobs:
     - name: Install Cygwin
       uses: cygwin/cygwin-install-action@v6
       with:
-        packages: curl git python39 python-setuptools-wheel
+        packages: >-
+          curl gcc-core git libcurl-devel libexpat-devel libiconv-devel
+          libssl-devel make python39 python-setuptools-wheel zlib-devel
         add-to-path: false  # No need to change $PATH outside the Cygwin environment.
 
     - name: Arrange for verbose output
       run: |
         # Arrange for verbose output but without shell environment setup details.
         echo 'set -x' >~/.bash_profile
 
+    - name: Build minimum supported Cygwin Git
+      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
+        hash -r
+        test "$(git version)" = 'git version 2.52.0'
+
     - name: Special configuration for Cygwin git
       run: |
         git config --global --add safe.directory "$(pwd)"