Run test_commit_msg_hook_success on more systems

This changes a default Windows skip of test_commit_msg_hook_success
to an xfail, and makes it more specific, expecting failure only
when either bash.exe is unavailable (definitely expected) or when
bash.exe is the WSL bash wrapper in System32, which fails for some
reason even though it's not at all clear it ought to.

This showcases the failures rather than skipping, and also lets the
test pass on Windows systems where bash.exe is something else,
including the Git Bash bash.exe that native Windows CI would use.

test/test_index.py

@@ -7,10 +7,14 @@
 
 from io import BytesIO
 import os
+import os.path as osp
+from pathlib import Path
 from stat import S_ISLNK, ST_MODE
+import shutil
 import tempfile
 from unittest import skipIf
-import shutil
+
+import pytest
 
 from git import (
     IndexFile,
@@ -23,6 +27,7 @@
     GitCommandError,
     CheckoutError,
 )
+from git.cmd import Git
 from git.compat import is_win
 from git.exc import HookExecutionError, InvalidGitRepositoryError
 from git.index.fun import hook_path
@@ -34,15 +39,22 @@
 from git.util import HIDE_WINDOWS_KNOWN_ERRORS, hex_to_bin
 from gitdb.base import IStream
 
-import os.path as osp
-from git.cmd import Git
+HOOKS_SHEBANG = "#!/usr/bin/env sh\n"
 
-from pathlib import Path
 
-HOOKS_SHEBANG = "#!/usr/bin/env sh\n"
+def _found_in(cmd, directory):
+    """Check if a command is resolved in a directory (without following symlinks)."""
+    path = shutil.which(cmd)
+    return path and Path(path).parent == Path(directory)
+
 
 is_win_without_bash = is_win and not shutil.which("bash.exe")
 
+is_win_with_wsl_bash = is_win and _found_in(
+    cmd="bash.exe",
+    directory=Path(os.getenv("WINDIR")) / "System32",
+)
+
 
 def _make_hook(git_dir, name, content, make_exec=True):
     """A helper to create a hook"""
@@ -910,7 +922,11 @@ def test_pre_commit_hook_fail(self, rw_repo):
         else:
             raise AssertionError("Should have caught a HookExecutionError")
 
-    @skipIf(HIDE_WINDOWS_KNOWN_ERRORS, "TODO: fix hooks execution on Windows: #703")
+    @pytest.mark.xfail(
+        is_win_without_bash or is_win_with_wsl_bash,
+        reason="Specifically seems to fail on WSL bash (in spite of #1399)",
+        raises=AssertionError,
+    )
     @with_rw_repo("HEAD", bare=True)
     def test_commit_msg_hook_success(self, rw_repo):
         commit_message = "commit default head by Frèderic Çaufl€"