manifest: call m1.load and m2.load before writing a subtree

As part of refactoring the manifest, certain test cases started failing because
writesubtrees was called with p1 and p2 manifests that had not been loaded (so
accessing m1._dirs resulted in an empty set). Let's call _load on these before
attempting to access _dirs.

This was caught by tests when future patches were applied.

mercurial/manifest.py

@@ -882,6 +882,8 b' class treemanifest(object):'
 
     def writesubtrees(self, m1, m2, writesubtree):
         self._load() # for consistency; should never have any effect here
+        m1._load()
+        m2._load()
         emptytree = treemanifest()
         for d, subm in self._dirs.iteritems():
             subp1 = m1._dirs.get(d, emptytree)._node