commit: avoid redundant scissor line with --cleanup=scissors -v

commit: avoid redundant scissor line with --cleanup=scissors -v

`git commit --cleanup=scissors -v` prints two scissors lines:
one at the start of the comment lines, and the other right before the
diff. This is redundant, and pushes the diff further down in the user's
editor than it needs to be.

Make wt_status_add_cut_line() remember if it has added a cut line before,
and avoid adding a redundant one.

Add a test for this.

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