Add a direct test of run_commit_hook

This has three benefits:

- run_commit_hook is public, being listed in git.index.fun.__all__,
  so it makes sense for it to have its own test.

- When investigating (future, or current xfail-covered) failure of
  functions that use run_commit_hook, it will be useful to compare
  the results of other tests that already do exist to that of a
  direct test of run_commit_hook.

- When changing which bash.exe run_commit_hook selects to use on
  Windows (including to ameliorate the limitation covered by the
  WinBashStatus.WslNoDistro xfail marks, or to attempt other
  possible changes suggested in #1745), or even just to investigate
  the possibility of doing so, it will make sense to add tests like
  this test but for more specific conditions or edge cases. Having
  this typical-case test to compare to should be helpful both for
  writing such tests and for efficiently verifying that the
  conditions they test are really the triggers for any failures.

test/test_index.py

@@ -32,7 +32,7 @@
     InvalidGitRepositoryError,
     UnmergedEntriesError,
 )
-from git.index.fun import hook_path
+from git.index.fun import hook_path, run_commit_hook
 from git.index.typ import BaseIndexEntry, IndexEntry
 from git.objects import Blob
 from test.lib import TestBase, fixture, fixture_path, with_rw_directory, with_rw_repo
@@ -991,6 +991,24 @@ class Mocked:
         rel = index._to_relative_path(path)
         self.assertEqual(rel, os.path.relpath(path, root))
 
+    @pytest.mark.xfail(
+        type(_win_bash_status) is WinBashStatus.Absent,
+        reason="Can't run a hook on Windows without bash.exe.",
+        rasies=HookExecutionError,
+    )
+    @pytest.mark.xfail(
+        type(_win_bash_status) is WinBashStatus.WslNoDistro,
+        reason="Currently uses the bash.exe of WSL, even with no WSL distro installed",
+        raises=HookExecutionError,
+    )
+    @with_rw_repo("HEAD", bare=True)
+    def test_run_commit_hook(self, rw_repo):
+        index = rw_repo.index
+        _make_hook(index.repo.git_dir, "fake-hook", "echo 'ran fake hook' >output.txt")
+        run_commit_hook("fake-hook", index)
+        output = Path(rw_repo.git_dir, "output.txt").read_text(encoding="utf-8")
+        self.assertEqual(output, "ran fake hook\n")
+
     @pytest.mark.xfail(
         type(_win_bash_status) is WinBashStatus.Absent,
         reason="Can't run a hook on Windows without bash.exe.",