Update dirname logic.

TBR=maruel@chromium.org,cmp@chromium.org
Review URL: https://codereview.chromium.org/11275219

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@166808 0039d316-1c4b-4281-b951-d872f2087c98

git-try

@@ -3,13 +3,17 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-base_dir=$(dirname "$0")
-
 # Test if this script is running under a MSys install.  If it is, we will
 # hardcode the path to Python where possible.
 OUTPUT="$(uname | grep 'MINGW')"
 MINGW=$?
 
+if [ $MINGW = 0 ]; then
+  base_dir="${0%\\*}"
+else
+  base_dir=$(dirname "$0")
+fi
+
 if [ -d "$base_dir/python_bin" -a $MINGW = 0 ]; then
   PYTHONDONTWRITEBYTECODE=1 exec "$base_dir/python_bin/python.exe" "$base_dir"/git_try.py "$@"
 else