dir: fix pattern matching on dirs

dir: fix pattern matching on dirs

Within match_pathname(), one successful matching category happens when
the pattern is equal to its non-wildcard prefix. At this point, we have
checked that the input 'pathname' matches the pattern up to the prefix
length, and then we subtraced that length from both 'patternlen' and
'namelen'.

In the case of a directory match, this prefix match should be
sufficient. However, the success condition only cared about _exact_
equality here. Instead, we should allow any path that agrees on this
prefix in the case of PATTERN_FLAG_MUSTBEDIR.

This case was not tested before because of the way unpack_trees() would
match a parent directory before visiting the contained paths. This
approach is changing, so we must change this comparison.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>