This optimises away the need to run anything in some common cases.
It's particularly useful on push; no need to push if the tracking branch
we just pulled is the same as the branch we're going to push.
The other uses of it can all be simplified using Git.Ref.base,
Git.Ref.under, and show.
In some cases, describe was being used to shorten the branch name
unnecessarily, and I instead pass the fully qualified name to git.
git-annex normally only runs the branch update once per run, for speed, but
since this fetches new remote git-annex tracking branches, they need to be
merged in after that fetch. An earlier call to Remote.byName was causing
the update to run before the fetch sometimes, but it could have been
anything. Just force the update to happen in the right place.
