This is similar to the Gerrit behavior in that we default to master,
unless the remote upstream is a branch head.

BUG=435702

Review URL: https://codereview.chromium.org/822503005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293807 0039d316-1c4b-4281-b951-d872f2087c98