merge: add scissors line on merge conflict

merge: add scissors line on merge conflict

This fixes a bug where the scissors line is placed after the Conflicts:
section, in the case where a merge conflict occurs and
commit.cleanup = scissors.

Next, if commit.cleanup = scissors is specified, don't produce a
scissors line in commit if one already exists in the MERGE_MSG file.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>