rebase: support --autosquash without -i

rebase: support --autosquash without -i

The rebase --autosquash option is quietly ignored when used without
--interactive (apart from preventing preemptive fast-forwarding and
triggering conflicts with apply backend options).

Change that to support --autosquash without --interactive, by dropping
its restriction to REBASE_INTERACTIVE_EXCPLICIT mode. When used this
way, auto-squashing is done without opening the todo list editor.

Drop the -i requirement from the --autosquash description, and amend
t3415-rebase-autosquash.sh to test the option and the rebase.autoSquash
config variable with and without -i.

Signed-off-by: Andy Koppe <andy.koppe@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>