fsmonitor: return invalidated cache-entry count on directory event

fsmonitor: return invalidated cache-entry count on directory event

Teach the refresh callback helper function for directory FSEvents to
return the number of cache-entries that were invalidated in response
to a directory event.

This will be used in a later commit to help determine if the observed
pathname in the FSEvent was a (possibly) case-incorrect directory
prefix (on a case-insensitive filesystem) of one or more actual
cache-entries.

If there exists at least one case-insensitive prefix match, then we
can assume that the directory is a (case-incorrect) prefix of at least
one tracked item rather than a completely unknown/untracked file or
directory.

Signed-off-by: Jeff Hostetler <jeffhostetler@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>