sequencer: always commit without editing when asked for

sequencer: always commit without editing when asked for

Previously, we only called run_git_commit() without EDIT_MSG when we also
passed in a default message.

However, an upcoming caller will want to commit without EDIT_MSG and
*without* a default message: to clean up fixup/squash comments in HEAD's
commit message.

Let's prepare for that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>