add-patch: allow disabling editing of hunks

add-patch: allow disabling editing of hunks

The "add-patch" mode allows the user to edit hunks to apply custom
changes. This is incompatible with a new `git history split` command
that we're about to introduce in a subsequent commit, so we need a way
to disable this mode.

Add a new flag to disable editing hunks.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>