pull: support rebased upstream + fetch + pull --rebase

pull: support rebased upstream + fetch + pull --rebase

You cannot do a "git pull --rebase" with a rebased upstream, if you have
already run "git fetch".  Try to behave as if the "git fetch" was not run.

In other words, find the fork point of the current branch, where
the tip of upstream branch used to be, and use it as the upstream
parameter of "git rebase".

This patch computes the fork point by walking the reflog to find the first
commit which is an ancestor of the current branch.  Maybe there are
smarter ways to compute it, but this is a straight forward implementation.

Signed-off-by: Santi Béjar <santi@agolina.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>