rebase -i: honor --rerere-autoupdate

rebase -i: honor --rerere-autoupdate

Interactive rebase was ignoring '--rerere-autoupdate'. Fix this by
reading it appropriate file when restoring the sequencer state for an
interactive rebase and passing '--rerere-autoupdate' to merge and
cherry-pick when rebasing with '--preserve-merges'.

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>