sequencer: use argv_array_pushf

This avoids a manual allocation calculation, and is shorter
to boot.

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

sequencer.c

@@ -396,18 +396,13 @@ static int run_git_commit(const char *defmsg, struct replay_opts *opts,
 {
 	struct argv_array array;
 	int rc;
-	char *gpg_sign;
 
 	argv_array_init(&array);
 	argv_array_push(&array, "commit");
 	argv_array_push(&array, "-n");
 
-	if (opts->gpg_sign) {
-		gpg_sign = xmalloc(3 + strlen(opts->gpg_sign));
-		sprintf(gpg_sign, "-S%s", opts->gpg_sign);
-		argv_array_push(&array, gpg_sign);
-		free(gpg_sign);
-	}
+	if (opts->gpg_sign)
+		argv_array_pushf(&array, "-S%s", opts->gpg_sign);
 	if (opts->signoff)
 		argv_array_push(&array, "-s");
 	if (!opts->edit) {