largefiles: fix _always for match overrides
Upcoming patches will speed dirstate.walk up by not filtering based on the
match function when match.always() is True. For that to work, match.always()
needs to be accurate. Previously it wasn't so for largefiles.
dirstate.walk: remove subrepo and .hg from results before step 3
An upcoming patch will speed dirstate.walk up by not querying the results dict
when it is empty. This ensures it is in some common cases.
This should be safe because subrepos and .hg aren't part of the dirstate.