treemanifest: separate flags for trees in memory and trees on disk

When we start writing tree manifests with one manifest revlog per
directory, it will still be nice to be able to run tests using tree
manifests in memory but writing to a flat manifest to a single
revlog. Let's break the current '_usetreemanifest' flag on the revlog
into '_treeinmem' and '_treeondisk'. Both are populated from the same
config, but after this change, one can temporarily hard-code
_treeinmem=True to see that tests still pass.

mercurial/manifest.py

@@ -761,11 +761,12 b' class manifest(revlog.revlog):'
             usemanifestv2 = opts.get('manifestv2', usemanifestv2)
         self._mancache = util.lrucachedict(cachesize)
         revlog.revlog.__init__(self, opener, "00manifest.i")
-        self._usetreemanifest = usetreemanifest
+        self._treeinmem = usetreemanifest
+        self._treeondisk = usetreemanifest
         self._usemanifestv2 = usemanifestv2
 
     def _newmanifest(self, data=''):
-        if self._usetreemanifest:
+        if self._treeinmem:
             return treemanifest('', data)
         return manifestdict(data)
 
@@ -782,7 +783,7 b' class manifest(revlog.revlog):'
         return md
 
     def readdelta(self, node):
-        if self._usemanifestv2 or self._usetreemanifest:
+        if self._usemanifestv2 or self._treeondisk:
             return self._slowreaddelta(node)
         r = self.rev(node)
         d = mdiff.patchtext(self.revdiff(self.deltaparent(r), r))
@@ -817,7 +818,7 b' class manifest(revlog.revlog):'
             return None, None
 
     def add(self, m, transaction, link, p1, p2, added, removed):
-        if (p1 in self._mancache and not self._usetreemanifest
+        if (p1 in self._mancache and not self._treeinmem
             and not self._usemanifestv2):
             # If our first parent is in the manifest cache, we can
             # compute a delta here using properties we know about the