rebase: silence "git checkout" for noop rebase

rebase: silence "git checkout" for noop rebase

When the branch to be rebased is already up to date, we
"git checkout" the branch, print an "up to date" message,
and end the rebase early. However, our checkout may print
"Switched to branch 'foo'" or "Already on 'foo'", even if
the user has asked for "--quiet".

We should avoid printing these messages at all, "--quiet" or
no. Since the rebase is a noop, this checkout can be seen as
optimizing out these other two checkout operations (that
happen in a real rebase):

  1. Moving to the detached HEAD to start the rebase; we
     always feed "-q" to checkout there, and instead rely on
     our own custom message (which respects --quiet).

  2. Finishing a rebase, where we move to the final branch.
     Here we actually use update-ref rather than
     git-checkout, and produce no messages.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>