wildmatch: make a special case for "*/" with FNM_PATHNAME

wildmatch: make a special case for "*/" with FNM_PATHNAME

Normally we need recursion for "*". In this case we know that it
matches everything until "/" so we can skip the recursion.

glibc, '*/*/*' on linux-2.6.git file list 2000 times
before:
wildmatch 8s 74513us
fnmatch   1s 97042us or 13.59% faster
after:
wildmatch 3s 521862us
fnmatch   3s 488616us or 99.06% slower

Same test with compat/fnmatch:
wildmatch 8s 110763us
fnmatch   2s 980845us or 36.75% faster
wildmatch 3s 522156us
fnmatch   1s 544487us or 43.85% slower

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>