fsmonitor--daemon: add pathname classification

fsmonitor--daemon: add pathname classification

Teach fsmonitor--daemon to classify relative and absolute
pathnames and decide how they should be handled.  This will
be used by the platform-specific backend to respond to each
filesystem event.

When we register for filesystem notifications on a directory,
we get events for everything (recursively) in the directory.
We want to report to clients changes to tracked and untracked
paths within the working directory proper.  We do not want to
report changes within the .git directory, for example.

This classification will be used in a later commit by the
different backends to classify paths as events are received.

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