avoid using sha1_to_hex output as printf format

We know that it should not contain any percent-signs, but
it's a good habit not to feed non-literals to printf
formatters.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/worktree.c

@@ -262,7 +262,7 @@ static int add_worktree(const char *path, const char *refname,
 	 */
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/HEAD", sb_repo.buf);
-	write_file(sb.buf, sha1_to_hex(null_sha1));
+	write_file(sb.buf, "%s", sha1_to_hex(null_sha1));
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/commondir", sb_repo.buf);
 	write_file(sb.buf, "../..");