merge-file: handle freopen() failure

Report the error if redirection of stderr to /dev/null failed.

This silences a compiler warning about ignoring the return value
of freopen() on Ubuntu 8.10.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-merge-file.c

@@ -51,8 +51,11 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, options, merge_file_usage, 0);
 	if (argc != 3)
 		usage_with_options(merge_file_usage, options);
-	if (quiet)
-		freopen("/dev/null", "w", stderr);
+	if (quiet) {
+		if (!freopen("/dev/null", "w", stderr))
+			return error("failed to redirect stderr to /dev/null: "
+				     "%s\n", strerror(errno));
+	}
 
 	for (i = 0; i < 3; i++) {
 		if (!names[i])