commit: unify logic to avoid multiple scissors lines when merging

commit: unify logic to avoid multiple scissors lines when merging

prepare_to_commit has some logic to figure out whether merge already
added a scissors line, and therefore it shouldn't add another. Now that
wt_status_add_cut_line has built-in state for whether it has
already added a previous line, just set that state instead, and then
remove that condition from subsequent calls to wt_status_add_cut_line.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>