Fix dirname logic for msys and not-msys.

git-cl

 #!/bin/sh
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # git-cl -- a git-command for integrating reviews on Rietveld
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
-base_dir=$(dirname "$0")
-
-# Uncomment this line if you never use gclient.
-# "$base_dir"/update_depot_tools
-
 # Test if this script is running under a MSys install.  If it is, we will
 # hardcode the path to Python where possible.
 OUTPUT="$(uname | grep 'MINGW')"
 MINGW=$?
 
+if [ $MINGW = 0 ]; then
+  base_dir="${0%\\*}"
+else
+  base_dir=$(dirname "$0")
+fi
+
+# Uncomment this line if you never use gclient.
+# "$base_dir"/update_depot_tools
+
 if [ -d "$base_dir/python_bin" -a $MINGW = 0 ]; then
   PYTHONDONTWRITEBYTECODE=1 exec "$base_dir/python_bin/python.exe" "$base_dir"/git_cl.py "$@"
 else
   PYTHONDONTWRITEBYTECODE=1 exec "$base_dir/git_cl.py" "$@"
 fi