treemanifests: make _loadlazy tolerate item not on _lazydirs

I'd like to clean up a few cases where we check for an item in _lazydirs before
calling _loadlazy - this will remove an extraneous dict lookup and make it
slightly more versatile.

Differential Revision: https://phab.mercurial-scm.org/D4842

mercurial/manifest.py

@@ -706,7 +706,9 class treemanifest(object):
         self._lazydirs = {}
 
     def _loadlazy(self, d):
-        path, node, readsubtree = self._lazydirs[d]
+        v = self._lazydirs.get(d)
+        if v:
+            path, node, readsubtree = v
         self._dirs[d] = readsubtree(path, node)
         del self._lazydirs[d]