rebase -i: teach --onto A...B syntax

rebase -i: teach --onto A...B syntax

When rewriting commits on a topic branch, sometimes it is easier to
compare the version of commits before and after the rewrite if they are
based on the same commit that forked from the upstream. An earlier commit
by Junio (fixed up by the previous commit) gives "--onto A...B" syntax to
rebase command, and rebases on top of the merge base between A and B;
teach the same to the interactive version, too.

Signed-off-by: しらいし ななこ <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>