fsmonitor: use modern match API
It seems like fsmonitor/__init__.py was based on a pretty old version
of dirstate.py. Let's copy over the changes from the following two
commits:
55c449345b10 (match: add isexact() method to hide internals, 2014-10-29)
3c346969c321 (dirstate: avoid match.files() in walk(), 2015-05-19)