treemanifest: make pending tree writes accessible from manifestlog

When writing a series of treemanifests, future trees may need to reference trees
that have already been written. It can't look in the pack file like normal
because the pack file hasn't been finalized yet. To make those written trees
accessible, let's put them in a temporary memory set that the treemanifestlog
knows to look in.

treemanifest/__init__.py

@@ -260,6 +260,7 @@ class treemanifestlog(manifest.manifestlog):
 class treeonlymanifestlog(object):
     def __init__(self, opener):
         self._opener = opener
+        self._memtrees = {}
 
     def __getitem__(self, node):
         return self.get('', node)
@@ -271,15 +272,25 @@ class treeonlymanifestlog(object):
         if node == nullid:
             return treemanifestctx(self, dir, node)
 
+        memtree = self._memtrees.get((dir, node))
+        if memtree is not None:
+            return memtree
+
         store = self.datastore
+
         if not store.getmissing([(dir, node)]):
             return treemanifestctx(self, dir, node)
         else:
             raise KeyError("tree node not found (%s, %s)" %
                            (dir, hex(node)))
 
+    def addmemtree(self, node, tree):
+        ctx = treemanifestctx(self, '', node)
+        ctx._data = tree
+        self._memtrees[('', node)] = ctx
+
     def clearcaches(self):
-        pass
+        self._memtrees.clear()
 
 class treemanifestctx(object):
     def __init__(self, manifestlog, dir, node):
@@ -433,6 +444,8 @@ class memtreemanifestctx(object):
             if nname == "":
                 node = nnode
 
+        if node is not None:
+            self._manifestlog.addmemtree(node, newtree)
         return node
 
 def serverreposetup(repo):