Always return a tuple from GIT.Capture

Review URL: http://codereview.chromium.org/555198

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@37685 0039d316-1c4b-4281-b951-d872f2087c98

scm.py

@@ -60,7 +60,7 @@ class GIT(object):
       return gclient_utils.CheckCall(c, in_directory, print_error)
     except gclient_utils.CheckCallError:
       if error_ok:
-        return ''
+        return ('', '')
       raise
 
   @staticmethod