Allow branch name to be the same as directory name for "git cl diff"
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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@293914 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools