dirstate: use match.prefix() instead of 'not match.anypats()'

It seems clearer to check for what it is than what it isn't.

mercurial/dirstate.py

@@ -985,7 +985,7 b' class dirstate(object):'
             # fast path -- filter the other way around, since typically files is
             # much smaller than dmap
             return [f for f in files if f in dmap]
-        if not match.anypats() and all(fn in dmap for fn in files):
+        if match.prefix() and all(fn in dmap for fn in files):
             # fast path -- all the values are known to be files, so just return
             # that
             return list(files)