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>

builtin/merge.c

@@ -920,6 +920,20 @@ static int suggest_conflicts(void)
 	filename = git_path_merge_msg(the_repository);
 	fp = xfopen(filename, "a");
 
+	/*
+	 * We can't use cleanup_mode because if we're not using the editor,
+	 * get_cleanup_mode will return COMMIT_MSG_CLEANUP_SPACE instead, even
+	 * though the message is meant to be processed later by git-commit.
+	 * Thus, we will get the cleanup mode which is returned when we _are_
+	 * using an editor.
+	 */
+	if (get_cleanup_mode(cleanup_arg, 1) == COMMIT_MSG_CLEANUP_SCISSORS) {
+	    fputc('\n', fp);
+	    wt_status_add_cut_line(fp);
+	    /* comments out the newline from append_conflicts_hint */
+	    fputc(comment_line_char, fp);
+	}
+
 	append_conflicts_hint(&the_index, &msgbuf);
 	fputs(msgbuf.buf, fp);
 	strbuf_release(&msgbuf);