Character class "xdigit" is the only one that hits 6 character limit
defined by CHAR_CLASS_MAX_LENGTH. All other character classes are 5
character long and therefore never caught by this.
This should make xdigit tests in t3070 pass on Windows.
Reported-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>