replace: use argv_array in export_object

This is a little more verbose, but will make it easier to
make parts of our command-line conditional (without
resorting to magic numbers or lots of NULLs to get an
appropriately sized argv array).

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

builtin/replace.c

@@ -193,15 +193,17 @@ static int replace_object(const char *object_ref, const char *replace_ref, int f
  */
 static void export_object(const unsigned char *sha1, const char *filename)
 {
-	const char *argv[] = { "--no-replace-objects", "cat-file", "-p", NULL, NULL };
-	struct child_process cmd = { argv };
+	struct child_process cmd = { NULL };
 	int fd;
 
 	fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0666);
 	if (fd < 0)
 		die_errno("unable to open %s for writing", filename);
 
-	argv[3] = sha1_to_hex(sha1);
+	argv_array_push(&cmd.args, "--no-replace-objects");
+	argv_array_push(&cmd.args, "cat-file");
+	argv_array_push(&cmd.args, "-p");
+	argv_array_push(&cmd.args, sha1_to_hex(sha1));
 	cmd.git_cmd = 1;
 	cmd.out = fd;