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)

hgext/fsmonitor/__init__.py

@@ -253,10 +253,10 b' def overridewalk(orig, self, match, subr'
     fresh_instance = False
 
     exact = skipstep3 = False
-    if matchfn == match.exact:  # match.exact
+    if match.isexact():  # match.exact
         exact = True
         dirignore = util.always  # skip step 2
-    elif match.files() and not match.anypats():  # match.match, no patterns
+    elif match.prefix():  # match.match, no patterns
         skipstep3 = True
 
     if not exact and self._checkcase: