builtin-mailinfo.c: use "ISO8859-1" instead of "latin1" as fallback encoding

Some platforms do not understand the character encoding "latin1" which is
another name for "ISO8859-1".  So use "ISO8859-1" instead which all tested
platforms understand.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-mailinfo.c

@@ -480,7 +480,7 @@ static const char *guess_charset(const struct strbuf *line, const char *target_c
 		if (is_utf8(line->buf))
 			return NULL;
 	}
-	return "latin1";
+	return "ISO8859-1";
 }
 
 static void convert_to_utf8(struct strbuf *line, const char *charset)