Teach rebase the --no-ff option.

Teach rebase the --no-ff option.

For git-rebase.sh, --no-ff is a synonym for --force-rebase.

For git-rebase--interactive.sh, --no-ff cherry-picks all the commits in
the rebased branch, instead of fast-forwarding over any unchanged commits.

--no-ff offers an alternative way to deal with reverted merges.  Instead of
"reverting the revert" you can use "rebase --no-ff" to recreate the branch
with entirely new commits (they're new because at the very least the
committer time is different).  This obviates the need to revert the
reversion, as you can re-merge the new topic branch directly.  Added an
addendum to revert-a-faulty-merge.txt describing the situation and how to
use --no-ff to handle it.

Signed-off-by: Marc Branchaud <marcnarc@xiplink.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>