manifest: add manifestlog.get to obtain subdirectory instances

manifest: add manifestlog.get to obtain subdirectory instances

Previously manifestlog only allowed obtaining root level manifests. Future
patches will need direct access to subdirectory manifests as part of changegroup
creation, so let's add a get() function that knows how to deal with
subdirectories.

--- a/mercurial/manifest.py	Wed Nov 02 17:33:31 2016 -0700
+++ b/mercurial/manifest.py	Wed Nov 02 17:24:06 2016 -0700
@@ -1265,23 +1265,41 @@
         """Retrieves the manifest instance for the given node. Throws a
         LookupError if not found.
         """
-        if node in self._mancache:
-            cachemf = self._mancache[node]
-            # The old manifest may put non-ctx manifests in the cache, so skip
-            # those since they don't implement the full api.
-            if (isinstance(cachemf, manifestctx) or
-                isinstance(cachemf, treemanifestctx)):
-                return cachemf
+        return self.get('', node)
 
-        if node not in self._revlog.nodemap:
-            raise LookupError(node, self._revlog.indexfile,
-                              _('no node'))
-        if self._treeinmem:
-            m = treemanifestctx(self._repo, '', node)
+    def get(self, dir, node):
+        """Retrieves the manifest instance for the given node. Throws a
+        LookupError if not found.
+        """
+        if dir:
+            if self._revlog._treeondisk:
+                dirlog = self._revlog.dirlog(dir)
+                if node not in dirlog.nodemap:
+                    raise LookupError(node, dirlog.indexfile,
+                                      _('no node'))
+                m = treemanifestctx(self._repo, dir, node)
+            else:
+                raise error.Abort(
+                        _("cannot ask for manifest directory '%s' in a flat "
+                          "manifest") % dir)
         else:
-            m = manifestctx(self._repo, node)
-        if node != revlog.nullid:
-            self._mancache[node] = m
+            if node in self._mancache:
+                cachemf = self._mancache[node]
+                # The old manifest may put non-ctx manifests in the cache, so
+                # skip those since they don't implement the full api.
+                if (isinstance(cachemf, manifestctx) or
+                    isinstance(cachemf, treemanifestctx)):
+                    return cachemf
+
+            if node not in self._revlog.nodemap:
+                raise LookupError(node, self._revlog.indexfile,
+                                  _('no node'))
+            if self._treeinmem:
+                m = treemanifestctx(self._repo, '', node)
+            else:
+                m = manifestctx(self._repo, node)
+            if node != revlog.nullid:
+                self._mancache[node] = m
         return m
 
     def add(self, m, transaction, link, p1, p2, added, removed):