Discover git.bat even if git_cl.py is executed using relative path

This allows git_cl.py to be used without adding depot_tools to PATH

BUG=
R=iannucci@chromium.org

Review URL: https://codereview.chromium.org/471413003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@289992 0039d316-1c4b-4281-b951-d872f2087c98

git_common.py

@@ -30,8 +30,9 @@ import threading
 
 import subprocess2
 
+ROOT = os.path.abspath(os.path.dirname(__file__))
 
-GIT_EXE = 'git.bat' if sys.platform.startswith('win') else 'git'
+GIT_EXE = ROOT+'\\git.bat' if sys.platform.startswith('win') else 'git'
 TEST_MODE = False
 
 FREEZE = 'FREEZE'