ci: also run the Windows test suite with BusyBox' ash

The BusyBox flavor of MinGit ships without an MSYS2 shell and instead
points `core.shell` at BusyBox' `ash.exe`. To keep that configuration
working, and as a step toward the longer-term goal of running Git for
Windows' test suite faster by using BusyBox instead of the MSYS2 Bash,
let's exercise the entire Windows test suite a second time using `ash`
as the POSIX shell.

This adds a `use-busybox` dimension to the `win test` matrix. The build
artifacts are shared between the two legs; only the shell used to run
the tests differs, so the extra coverage is comparatively cheap.

The minimal Git for Windows SDK used in CI does not include the BusyBox
applets, so `ash.exe` is fetched from the full SDK. It is then wired in
via `GIT_TEST_SHELL_PATH` and `SHELL` (so that `git.exe` and the test
framework's BUSYBOX prerequisite pick it up) as well as `SHELL_PATH` and
`TEST_SHELL_PATH` (so that the test scripts themselves are run by it).

Assisted-by: Opus 4.8
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/main.yml

@@ -135,8 +135,9 @@ jobs:
       fail-fast: false
       matrix:
         nr: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        use-busybox: [false, true]
     concurrency:
-      group: windows-test-${{ matrix.nr }}-${{ github.ref }}
+      group: windows-test-${{ matrix.nr }}-${{ matrix.use-busybox }}-${{ github.ref }}
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - name: download tracked files and build artifacts
@@ -148,6 +149,19 @@ jobs:
       shell: bash
       run: tar xf artifacts.tar.gz && tar xf tracked.tar.gz
     - uses: git-for-windows/setup-git-for-windows-sdk@v2
+    - name: download BusyBox' ash.exe and use it as the POSIX shell
+      if: matrix.use-busybox == true
+      shell: bash
+      run: |
+        curl -fLo /mingw64/bin/ash.exe \
+          https://github.com/git-for-windows/git-sdk-64/raw/c7bf8b60df36106efa91fc0f68034bcef4896bfb/mingw64/libexec/busybox/ash.exe &&
+        ash="$(cygpath -aw /mingw64/bin/ash.exe)" &&
+        echo "GIT_TEST_SHELL_PATH=$ash" >>$GITHUB_ENV &&
+        echo "SHELL=$ash" >>$GITHUB_ENV &&
+        echo "SHELL_PATH=$ash" >>$GITHUB_ENV &&
+        echo "TEST_SHELL_PATH=$ash" >>$GITHUB_ENV &&
+        echo "SHELL_PATH=$ash" >>GIT-BUILD-OPTIONS &&
+        echo "TEST_SHELL_PATH=$ash" >>GIT-BUILD-OPTIONS
     - name: test
       shell: bash
       run: . /etc/profile && ci/run-test-slice.sh $((${{matrix.nr}} + 1)) 10
@@ -159,7 +173,7 @@ jobs:
       if: failure() && env.FAILED_TEST_ARTIFACTS != ''
       uses: actions/upload-artifact@v7
       with:
-        name: failed-tests-windows-${{ matrix.nr }}
+        name: failed-tests-windows-${{ matrix.nr }}-${{ matrix.use-busybox }}
         path: ${{env.FAILED_TEST_ARTIFACTS}}
   vs-build:
     name: win+VS build