mailinfo: fix 'fatal: cannot convert from utf-8 to utf-8'

For some reason, I got this error message.  Maybe it does not make sense,
but then we should not really try to convert the text when it is not
necessary.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-mailinfo.c

@@ -529,6 +529,8 @@ static void convert_to_utf8(char *line, const char *charset)
 			return;
 	}
 
+	if (!strcmp(metainfo_charset, charset))
+		return;
 	out = reencode_string(line, metainfo_charset, charset);
 	if (!out)
 		die("cannot convert from %s to %s\n",