Add changes for new branches in GitPoller

With the new branches=True argument for GitPoller to watch all
branches, _process_changes() does nothing if a new branch is pushed. The
only time it adds changes to the database is when a new commit is pushed
on an existing branch. This is unfortunate because this means in order
to have buildbot build all git branches, one must first push the branch,
wait for GitPoller to detect it, and then push new commits on the
branch.

http://trac.buildbot.net/ticket/2837
Log which branch GitPoller is fetching

When GitPoller is set up to watch multiple branches, this is useful
information for debugging.
Print detailed information if _dovccmd() in GitPoller fails

Otherwise the outputted information often isn't sufficient to debug the
problem.
Exclude all commits we already know about