If a merge can be fast-forwarded then make sure that we still edit the
commit message if the user specifies -c. The implementation follows the
same pattern that is used for ordinary rewords that are fast-forwarded.
Signed-off-by: NPhillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>