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>

builtin/commit.c

@@ -926,7 +926,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		if (whence != FROM_COMMIT) {
 			if (cleanup_mode == COMMIT_MSG_CLEANUP_SCISSORS &&
 				!merge_contains_scissors)
-				wt_status_add_cut_line(s->fp);
+				wt_status_add_cut_line(s);
 			status_printf_ln(
 				s, GIT_COLOR_NORMAL,
 				whence == FROM_MERGE ?
@@ -947,7 +947,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 			status_printf(s, GIT_COLOR_NORMAL, hint_cleanup_all, comment_line_char);
 		else if (cleanup_mode == COMMIT_MSG_CLEANUP_SCISSORS) {
 			if (whence == FROM_COMMIT && !merge_contains_scissors)
-				wt_status_add_cut_line(s->fp);
+				wt_status_add_cut_line(s);
 		} else /* COMMIT_MSG_CLEANUP_SPACE, that is. */
 			status_printf(s, GIT_COLOR_NORMAL, hint_cleanup_space, comment_line_char);
 

t/t7502-commit-porcelain.sh

@@ -736,6 +736,11 @@ test_expect_success 'message shows date when it is explicitly set' '
 	  .git/COMMIT_EDITMSG
 '
 
+test_expect_success 'message does not have multiple scissors lines' '
+	git commit --cleanup=scissors -v --allow-empty -e -m foo &&
+	test $(grep -c -e "--- >8 ---" .git/COMMIT_EDITMSG) -eq 1
+'
+
 test_expect_success AUTOIDENT 'message shows committer when it is automatic' '
 
 	echo >>negative &&

wt-status.c

@@ -1107,12 +1107,15 @@ void wt_status_append_cut_line(struct strbuf *buf)
 	strbuf_add_commented_lines(buf, explanation, strlen(explanation), comment_line_char);
 }
 
-void wt_status_add_cut_line(FILE *fp)
+void wt_status_add_cut_line(struct wt_status *s)
 {
 	struct strbuf buf = STRBUF_INIT;
 
+	if (s->added_cut_line)
+		return;
+	s->added_cut_line = 1;
 	wt_status_append_cut_line(&buf);
-	fputs(buf.buf, fp);
+	fputs(buf.buf, s->fp);
 	strbuf_release(&buf);
 }
 
@@ -1143,11 +1146,12 @@ static void wt_longstatus_print_verbose(struct wt_status *s)
 	 * file (and even the "auto" setting won't work, since it
 	 * will have checked isatty on stdout). But we then do want
 	 * to insert the scissor line here to reliably remove the
-	 * diff before committing.
+	 * diff before committing, if we didn't already include one
+	 * before.
 	 */
 	if (s->fp != stdout) {
 		rev.diffopt.use_color = 0;
-		wt_status_add_cut_line(s->fp);
+		wt_status_add_cut_line(s);
 	}
 	if (s->verbose > 1 && s->committable) {
 		/* print_updated() printed a header, so do we */

wt-status.h

@@ -130,6 +130,7 @@ struct wt_status {
 	int rename_score;
 	int rename_limit;
 	enum wt_status_format status_format;
+	unsigned char added_cut_line; /* boolean */
 	struct wt_status_state state;
 	struct object_id oid_commit; /* when not Initial */
 
@@ -147,7 +148,7 @@ struct wt_status {
 
 size_t wt_status_locate_end(const char *s, size_t len);
 void wt_status_append_cut_line(struct strbuf *buf);
-void wt_status_add_cut_line(FILE *fp);
+void wt_status_add_cut_line(struct wt_status *s);
 void wt_status_prepare(struct repository *r, struct wt_status *s);
 void wt_status_print(struct wt_status *s);
 void wt_status_collect(struct wt_status *s);