Use bash to open extensionless hooks on windows

Fix gitpython-developers#971.

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.

os.name [seems to be the best way to check for Windows][1] and it should
certainly ensure we don't do this on other platforms.

[1]: https://stackoverflow.com/questions/1325581/how-do-i-check-if-im-running-on-windows-in-python

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,
@@ -76,6 +77,10 @@ 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 +94,15 @@ 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 os.name == "nt" 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,