add-patch: update hunk splitability after editing

add-patch: update hunk splitability after editing

If, when the user edits a hunk, they change deletion lines into
context lines or vice versa, then the number of hunks that the edited
hunk can be split into may differ from the unedited hunk. This means
that so we should recalculate `hunk->splittable_into` after the hunk
has been edited. In practice users are unlikely to hit this bug as it
is doubtful that a user who has edited a hunk will split it afterwards.

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