Fix rev-parse for older git versions

pre_commit/git.py

@@ -31,16 +31,13 @@ def get_root():
 
 
 def get_git_dir(git_root):
-    def _git_dir(opt):
-        return os.path.normpath(os.path.join(
-            git_root,
-            cmd_output('git', 'rev-parse', opt, cwd=git_root)[1].strip(),
-        ))
-
-    try:
-        return _git_dir('--git-common-dir')
-    except CalledProcessError:  # pragma: no cover (git < 2.5)
-        return _git_dir('--git-dir')
+    opts = ('--git-common-dir', '--git-dir')
+    _, out, _ = cmd_output('git', 'rev-parse', *opts, cwd=git_root)
+    for line, opt in zip(out.splitlines(), opts):
+        if line != opt:  # pragma: no branch (git < 2.5)
+            return os.path.normpath(os.path.join(git_root, line))
+    else:
+        raise AssertionError('unreachable: no git dir')
 
 
 def get_remote_url(git_root):