Use bash to open extensionless hooks on windows

Fix #971. Partly resolve #703.

If the hook doesn't have a file extension, then Windows won't know how
to run it and you'll get "[WinError 193] %1 is not a valid Win32
application". It's very likely that it's a shell script of some kind, so
use bash.exe (commonly installed via Windows Subsystem for Linux). We
don't want to run all hooks with bash because they could be .bat files.

Update tests to get several hook ones working. More work necessary to
get commit-msg hook working. The hook writes to the wrong file because
it's not using forward slashes in the path:
C:\Users\idbrii\AppData\Local\Temp\bare_test_commit_msg_hook_successy5fo00du\CUsersidbriiAppDataLocalTempbare_test_commit_msg_hook_successy5fo00duCOMMIT_EDITMSG

git/index/fun.py

@@ -3,6 +3,7 @@
 # NOTE: Autodoc hates it if this is a docstring
 
 from io import BytesIO
+from pathlib import Path
 import os
 from stat import (
     S_IFDIR,
@@ -21,6 +22,7 @@
     force_text,
     force_bytes,
     is_posix,
+    is_win,
     safe_decode,
 )
 from git.exc import (
@@ -76,6 +78,10 @@ def hook_path(name: str, git_dir: PathLike) -> str:
     return osp.join(git_dir, 'hooks', name)
 
 
+def _has_file_extension(path):
+    return osp.splitext(path)[1]
+
+
 def run_commit_hook(name: str, index: 'IndexFile', *args: str) -> None:
     """Run the commit hook of the given name. Silently ignores hooks that do not exist.
     :param name: name of hook, like 'pre-commit'
@@ -89,8 +95,15 @@ def run_commit_hook(name: str, index: 'IndexFile', *args: str) -> None:
     env = os.environ.copy()
     env['GIT_INDEX_FILE'] = safe_decode(str(index.path))
     env['GIT_EDITOR'] = ':'
+    cmd = [hp]
     try:
-        cmd = subprocess.Popen([hp] + list(args),
+        if is_win and not _has_file_extension(hp):
+            # Windows only uses extensions to determine how to open files
+            # (doesn't understand shebangs). Try using bash to run the hook.
+            relative_hp = Path(hp).relative_to(index.repo.working_dir).as_posix()
+            cmd = ["bash.exe", relative_hp]
+
+        cmd = subprocess.Popen(cmd + list(args),
                                env=env,
                                stdout=subprocess.PIPE,
                                stderr=subprocess.PIPE,

test/test_index.py

@@ -13,6 +13,7 @@
 )
 import tempfile
 from unittest import skipIf
+import shutil
 
 from git import (
     IndexFile,
@@ -52,8 +53,9 @@
 
 HOOKS_SHEBANG = "#!/usr/bin/env sh\n"
 
+is_win_without_bash = is_win and not shutil.which('bash.exe')
+
 
-@skipIf(HIDE_WINDOWS_KNOWN_ERRORS, "TODO: fix hooks execution on Windows: #703")
 def _make_hook(git_dir, name, content, make_exec=True):
     """A helper to create a hook"""
     hp = hook_path(name, git_dir)
@@ -881,7 +883,7 @@ def test_pre_commit_hook_fail(self, rw_repo):
         try:
             index.commit("This should fail")
         except HookExecutionError as err:
-            if is_win:
+            if is_win_without_bash:
                 self.assertIsInstance(err.status, OSError)
                 self.assertEqual(err.command, [hp])
                 self.assertEqual(err.stdout, '')
@@ -896,6 +898,7 @@ 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")
     @with_rw_repo('HEAD', bare=True)
     def test_commit_msg_hook_success(self, rw_repo):
         commit_message = "commit default head by Frèderic Çaufl€"
@@ -920,7 +923,7 @@ def test_commit_msg_hook_fail(self, rw_repo):
         try:
             index.commit("This should fail")
         except HookExecutionError as err:
-            if is_win:
+            if is_win_without_bash:
                 self.assertIsInstance(err.status, OSError)
                 self.assertEqual(err.command, [hp])
                 self.assertEqual(err.stdout, '')