Allow the branch name updated by sync-webkit-git.py to be overriden using the git config in the WebKit directory.
Allow the branch name updated by sync-webkit-git.py to be overriden using the
git config in the WebKit directory.

My scenario is that I have the same git repository (using git-new-workdir) under
two different chromium enlistments. (Yes, even with git, I find it useful to
have more than one enlistment at times.)

BUG=None
TEST=Did a run of the tool without the config set and with it set to an
     existing branch and a non-existing branch.

Review URL: http://codereview.chromium.org/3239002

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@57690 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser