Resolve Windows hook Bash through PATH (#2198)

<!-- agent -->
Extensionless commit hooks on Windows were launched through a bare bash.exe.
Windows CreateProcess searches system directories before PATH for bare
executable names, so the WSL launcher in System32 could preempt Git for Windows
Bash and make every commit fail when no WSL distribution was installed.

A regression test mocks PATH resolution and demonstrates that the hook command
previously remained bare instead of using the resolved executable. Resolve
bash.exe with shutil.which before spawning, while retaining the bare-name
fallback when PATH has no Bash so existing WSL-based behavior remains available.

This matches Git for Windows v2.55.0.windows.3, whose git var GIT_SHELL_PATH
reports the absolute PATH-selected sh.exe. It also follows gix-command's
principle of resolving a Git-associated Windows shell before falling back to a
bare executable name.

Assisted-by: GPT 5.6
Co-authored-by: GPT 5.6 <codex@openai.com>

Author: Byron

git/index/fun.py

@@ -79,6 +79,21 @@ def _has_file_extension(path: str) -> str:
     return osp.splitext(path)[1]
 
 
+def _which_from_path(command: str) -> Union[str, None]:
+    """Resolve an executable 'command' from PATH without considering the current directory."""
+    cwd = osp.normcase(osp.abspath(os.curdir))
+    for directory in os.get_exec_path():
+        if not directory:
+            continue
+        directory = osp.abspath(directory)
+        if osp.normcase(directory) == cwd:
+            continue
+        candidate = osp.join(directory, command)
+        if osp.isfile(candidate) and os.access(candidate, os.X_OK):
+            return candidate
+    return None
+
+
 def run_commit_hook(name: str, index: "IndexFile", *args: str) -> None:
     """Run the commit hook of the given name. Silently ignore hooks that do not exist.
 
@@ -112,7 +127,11 @@ def run_commit_hook(name: str, index: "IndexFile", *args: str) -> None:
                 # an absolute path in this form, although a relative path is preferable
                 # because it also works with the Windows Subsystem for Linux wrapper.
                 bash_hp = hp
-            cmd = ["bash.exe", Path(bash_hp).as_posix()]
+            # Resolve through PATH before spawning. On Windows, CreateProcess searches
+            # the current and system directories before PATH for a bare executable name,
+            # which can otherwise select an impostor or the WSL launcher instead of Git
+            # for Windows' Bash.
+            cmd = [_which_from_path("bash.exe") or "bash.exe", Path(bash_hp).as_posix()]
 
         process = safer_popen(
             cmd + list(args),

test/test_index.py

@@ -1128,16 +1128,20 @@ def test_run_commit_hook_outside_worktree_on_windows(self, rw_dir):
         repo = Repo.init(root / "repo")
         hooks_dir = root / "hooks"
         _make_hook(root, "fake-hook", "exit 0")
+        bash = root / "git" / "bin" / "bash.exe"
         with repo.config_writer() as writer:
             writer.set_value("core", "hooksPath", str(hooks_dir))
 
-        with mock.patch("git.index.fun.sys.platform", "win32"):
+        with mock.patch("git.index.fun.sys.platform", "win32"), mock.patch(
+            "git.index.fun._which_from_path", return_value=str(bash)
+        ) as which:
             with mock.patch("git.index.fun.safer_popen") as popen, mock.patch("git.index.fun.handle_process_output"):
                 popen.return_value.returncode = 0
                 run_commit_hook("fake-hook", repo.index)
 
+        which.assert_called_once_with("bash.exe")
         command = popen.call_args[0][0]
-        self.assertEqual(command, ["bash.exe", "../hooks/fake-hook"])
+        self.assertEqual(command, [str(bash), "../hooks/fake-hook"])
 
     @ddt.data((False,), (True,))
     @with_rw_directory