diff: trivial fix for --output file error message

The option argument is either after the equal sign in --output=... or in
the next command-line argument. optarg is the reliable way to access it.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -3310,7 +3310,7 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 	else if ((argcount = parse_long_opt("output", av, &optarg))) {
 		options->file = fopen(optarg, "w");
 		if (!options->file)
-			die_errno("Could not open '%s'", arg + strlen("--output="));
+			die_errno("Could not open '%s'", optarg);
 		options->close_file = 1;
 		return argcount;
 	} else