lstat_cache: guard against full match of length of 'name' parameter

lstat_cache: guard against full match of length of 'name' parameter

longest_path_match() in symlinks.c does exactly what it's name says,
but in some cases that match can be too long, since the
has_*_leading_path() functions assumes that the match will newer be as
long as the name string given to the function.

fix this by adding an extra if test which checks if the match length
is equal to the 'len' parameter.

Signed-off-by: Junio C Hamano <gitster@pobox.com>