context: add a getfileset() method so fewer things need fileset directly

Allows me to break an import cycle in a subsequent change.

mercurial/context.py

@@ -12,6 +12,7 import match as matchmod
 import os, errno, stat
 import obsolete as obsmod
 import repoview
+import fileset
 
 propertycache = util.propertycache
 
@@ -79,6 +80,9 class basectx(object):
     def mutable(self):
         return self.phase() > phases.public
 
+    def getfileset(self, expr):
+        return fileset.getfileset(self, expr)
+
     def obsolete(self):
         """True if the changeset is obsolete"""
         return self.rev() in obsmod.getrevs(self._repo, 'obsolete')