manifest: make revlog verification optional

This patches adds an parameter to manifestlog.get() to disable hash checking.
This will be used in an upcoming patch to support treemanifestctx reading
sub-trees without loading them from the revlog. (This is already supported but
does not go through the manifestlog.get() code path)

mercurial/manifest.py

@@ -1278,9 +1278,12 b' class manifestlog(object):'
         """
         return self.get('', node)
 
-    def get(self, dir, node):
+    def get(self, dir, node, verify=True):
         """Retrieves the manifest instance for the given node. Throws a
         LookupError if not found.
+
+        `verify` - if True an exception will be thrown if the node is not in
+                   the revlog
         """
         if node in self._dirmancache.get(dir, ()):
             cachemf = self._dirmancache[dir][node]
@@ -1292,6 +1295,7 b' class manifestlog(object):'
 
         if dir:
             if self._revlog._treeondisk:
+                if verify:
                 dirlog = self._revlog.dirlog(dir)
                 if node not in dirlog.nodemap:
                     raise LookupError(node, dirlog.indexfile,
@@ -1302,6 +1306,7 b' class manifestlog(object):'
                         _("cannot ask for manifest directory '%s' in a flat "
                           "manifest") % dir)
         else:
+            if verify:
             if node not in self._revlog.nodemap:
                 raise LookupError(node, self._revlog.indexfile,
                                   _('no node'))