ls-files: fix "--format" output of relative paths

Fix a bug introduced with the "--format" option in
ce74de93 (ls-files: introduce "--format" option, 2022-07-23),
where relative paths were computed using the output buffer,
which could lead to random garbage data in the output.

Signed-off-by: Adam Johnson <me@adamj.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/ls-files.c

@@ -89,12 +89,15 @@ static void write_name(const char *name)
 
 static void write_name_to_buf(struct strbuf *sb, const char *name)
 {
-	const char *rel = relative_path(name, prefix_len ? prefix : NULL, sb);
+	struct strbuf buf = STRBUF_INIT;
+	const char *rel = relative_path(name, prefix_len ? prefix : NULL, &buf);
 
 	if (line_terminator)
 		quote_c_style(rel, sb, NULL, 0);
 	else
 		strbuf_addstr(sb, rel);
+
+	strbuf_release(&buf);
 }
 
 static const char *get_tag(const struct cache_entry *ce, const char *tag)

t/t3013-ls-files-format.sh

@@ -54,6 +54,22 @@ test_expect_success 'git ls-files --format path v.s. -s' '
 	test_cmp expect actual
 '
 
+test_expect_success 'git ls-files --format with relative path' '
+	cat >expect <<-\EOF &&
+	../o1.txt
+	../o2.txt
+	../o3.txt
+	../o4.txt
+	../o5.txt
+	../o6.txt
+	EOF
+	mkdir sub &&
+	cd sub &&
+	git ls-files --format="%(path)" ":/" >../actual &&
+	cd .. &&
+	test_cmp expect actual
+'
+
 test_expect_success 'git ls-files --format with -m' '
 	echo change >o1.txt &&
 	cat >expect <<-\EOF &&