Allow branch name to be the same as directory name for "git cl diff"

Remove the ambiguity between revision and filename (or directory name)
when using command "git cl diff".

BUG=454032

Review URL: https://codereview.chromium.org/889093002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293914 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -2785,9 +2785,9 @@ def CMDdiff(parser, args):
     if rtn != 0:
       return rtn
 
-    # Switch back to starting brand and diff against the temporary
+    # Switch back to starting branch and diff against the temporary
     # branch containing the latest rietveld patch.
-    subprocess2.check_call(['git', 'diff', TMP_BRANCH, branch])
+    subprocess2.check_call(['git', 'diff', TMP_BRANCH, branch, '--'])
   finally:
     RunGit(['checkout', '-q', branch])
     RunGit(['branch', '-D', TMP_BRANCH])