manifest: change treemanifestctx to construct subtrees from the manifestlog

Previously, treemanifestctx would directly construct its subtrees. By making it
get the subtrees through manifestlog.get() we consolidate all treemanifestctx
creation into manifestlog.get() and therefore extensions that need to wrap
manifestctx creation (like narrow-hg) can intercept manifestctxs at that single
place.

This also means fetching subtrees will take advantage of the manifestlog ctx
cache now, which it did not before.

mercurial/manifest.py

@@ -1489,7 +1489,10 b' class treemanifestctx(object):'
                 def gettext():
                     return rl.revision(self._node)
                 def readsubtree(dir, subm):
-                    return treemanifestctx(self._repo, dir, subm).read()
+                    # Set verify to False since we need to be able to create
+                    # subtrees for trees that don't exist on disk.
+                    return self._repo.manifestlog.get(dir, subm,
+                                                      verify=False).read()
                 m.read(gettext, readsubtree)
                 m.setnode(self._node)
                 self._data = m
@@ -1531,7 +1534,7 b' class treemanifestctx(object):'
         else:
             # Need to perform a slow delta
             r0 = revlog.deltaparent(revlog.rev(self._node))
-            m0 = treemanifestctx(self._repo, self._dir, revlog.node(r0)).read()
+            m0 = self._repo.manifestlog.get(self._dir, revlog.node(r0)).read()
             m1 = self.read()
             md = treemanifest(dir=self._dir)
             for f, ((n0, fl0), (n1, fl1)) in m0.diff(m1).iteritems():