wildmatch: fix "**" special case

"**" is adjusted to only be effective when surrounded by slashes, in
40bbee0 (wildmatch: adjust "**" behavior - 2012-10-15). Except that
the commit did it wrong:

1. when it checks for "the preceding slash unless ** is at the
   beginning", it compares to wrong pointer. It should have compared
   to the beginning of the pattern, not the text.

2. prev_p points to the character before "**", not the first "*". The
   correct comparison must be "prev_p < pattern" or
   "prev_p + 1 == pattern", not "prev_p == pattern".

3. The pattern must be surrounded by slashes unless it's at the
   beginning or the end of the pattern. We do two checks: one for the
   preceding slash and one the trailing slash. Both checks must be
   met. The use of "||" is wrong.

This patch fixes all above.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
compat/fnmatch: respect NO_FNMATCH* even on glibc

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
wildmatch: replace variable 'special' with better named ones

'special' is too generic and is used for two different purposes.
Replace it with 'match_slash' to indicate "**" pattern and 'negated'
for "[!...]" and "[^...]".

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