rebase: extend --signoff support

rebase: extend --signoff support

Allow --signoff to be used with --interactive and --merge. In
interactive mode only commits marked to be picked, edited or reworded
will be signed off.

The main motivation for this patch was to allow one to run 'git rebase
--exec "make check" --signoff' which is useful when preparing a patch
series for publication and is more convenient than doing the signoff
with another --exec command.

This change also allows --root without --onto to work with --signoff
as well (--root with --onto was already supported).

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>