builtin rebase: try to fast forward when possible

builtin rebase: try to fast forward when possible

In this commit, we add support to fast forward.

Note: we will need the merge base later, therefore the call to
can_fast_forward() really needs to be the first one when testing whether
we can skip the rebase entirely (otherwise, it would make more sense to
skip the possibly expensive operation if, say, running an interactive
rebase).

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>