fileset: add class to host special handling of initial subset

Future patches will add a function to switch mctx.ctx object so that we can
forcibly evaluate a fileset expression in a specified revision. For example,
new "revs()" function will be used to match predicate agains another revision

  $ hg revert 'set:revs(42, added())'

fullmatchctx class is similar to revset.fullreposet. It will allow us to
recalculate the subset only if it is not filtered yet.

mercurial/fileset.py

@@ -498,6 +498,12 b' class matchctx(object):'
     def narrow(self, files):
         return matchctx(self.ctx, self.filter(files), self._status)
 
+class fullmatchctx(matchctx):
+    """A match context where any files in any revisions should be valid"""
+
+    def __init__(self, ctx, subset=None, status=None):
+        super(fullmatchctx, self).__init__(ctx, subset, status)
+
 def _intree(funcs, tree):
     if isinstance(tree, tuple):
         if tree[0] == 'func' and tree[1][0] == 'symbol':
@@ -529,7 +535,7 b' def getfileset(ctx, expr):'
         status = None
         subset = list(ctx.walk(ctx.match([])))
 
-    return getset(matchctx(ctx, subset, status), tree)
+    return getset(fullmatchctx(ctx, subset, status), tree)
 
 def prettyformat(tree):
     return parser.prettyformat(tree, ('string', 'symbol'))