Maintain fully qualified tracking branches

When running repo branch, the git merge line (in many circumstances)
is set to the revision of the project specified in the manifest.  If
this is a branch name that is not fully-qualified, we will end up with
something like "merge = master" instead of "merge = refs/heads/master".
This change examines the revision if we are going to use that and
changes branch short names to fully qualified branch names.

Change-Id: Ie1be94fb8d45df8eeac44a47f729a3819a05fa81