fsmonitor: remove custom loop from non-directory path handler

fsmonitor: remove custom loop from non-directory path handler

Refactor the code that handles refresh events for pathnames that do
not contain a trailing slash.  Instead of using a custom loop to try
to scan the index and detect if the FSEvent named a file or might be a
directory prefix, use the recently created helper function to do that.

Also update the comments to describe what and why we are doing this.

On platforms that DO NOT annotate FS events with a trailing
slash, if we fail to find an exact match for the pathname
in the index, we do not know if the pathname represents a
directory or simply an untracked file.  Pretend that the pathname
is a directory and try again before assuming it is an untracked
file.

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