manifest: make manifestlog use it's own cache

As we start to make manifestlog the primary manifest source, the dependency on
manifest.manifest will cause circular dependency problems. Let's break this
dependency by making manifestlog use it's own cache. In a near future patch we
will remove the previous manifest cache so we're not duplicating it.

mercurial/manifest.py

@@ -1255,10 +1255,12 b' class manifestlog(object):'
         self._repo = repo
 
         usetreemanifest = False
+        cachesize = 4
 
         opts = getattr(opener, 'options', None)
         if opts is not None:
             usetreemanifest = opts.get('treemanifest', usetreemanifest)
+            cachesize = opts.get('manifestcachesize', cachesize)
         self._treeinmem = usetreemanifest
 
         self._oldmanifest = repo._constructmanifest()
@@ -1266,15 +1268,9 b' class manifestlog(object):'
 
         # A cache of the manifestctx or treemanifestctx for each directory
         self._dirmancache = {}
+        self._dirmancache[''] = util.lrucachedict(cachesize)
 
-        # We'll separate this into it's own cache once oldmanifest is no longer
-        # used
-        self._mancache = self._oldmanifest._mancache
-        self._dirmancache[''] = self._mancache
-
-        # A future patch makes this use the same config value as the existing
-        # mancache
-        self.cachesize = 4
+        self.cachesize = cachesize
 
     def __getitem__(self, node):
         """Retrieves the manifest instance for the given node. Throws a