rebase: honor --rerere-autoupdate

rebase: honor --rerere-autoupdate

Rebase accepts '--rerere-autoupdate' as an option but only honors it
if '-m' is also given. Fix it for a non-interactive rebase by passing
on the option to 'git am' and 'git cherry-pick'. Rework the tests so
that they can be used for each rebase flavor and extend them.

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