webkitpy: Fix branch name/ref inconsistency in git SCM module

Git._branch_tracking_remote_master() was returning a branch ref instead
of a local branch name. This was causing the module to erroneously detect
the repo as unclean.

BUG=305602
NOTRY=TRUE

Review URL: https://codereview.chromium.org/26684002

git-svn-id: svn://svn.chromium.org/blink/trunk@159273 bbb929c8-8fbe-4397-9dbb-9b2b20218538