rebase: write a reflog entry when finishing

rebase: write a reflog entry when finishing

When we finish a rebase, our detached HEAD is at the final
result. We update the original branch ref with this result,
and then point the HEAD symbolic ref at the updated branch.
We write a reflog for the branch update, but not for the
update of HEAD.

Because we're already at the final result on the detached
HEAD, moving to the branch actually doesn't change our
commit sha1 at all. So in that sense, a reflog entry would
be pointless.

However, humans do read reflogs, and an entry saying "rebase
finished: returning to refs/heads/master" can be helpful in
understanding what is going on.

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