Update dirname logic.

git-try

Index: git-try
===================================================================
--- git-try	(revision 166792)
+++ git-try	(working copy)
@@ -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