histedit: don't cleanup nodes already disposed of

If something else took care of these temporary nodes, we don't need to do
anything about it. This less liberal usage of pruning through cleanup nodes
will help us further cleanup on the road to explicitly tracks folds.

hgext/histedit.py

@@ -1202,6 +1202,7 b' def _finishhistedit(ui, repo, state, fm)'
         mapping = {}
 
     for n in tmpnodes:
+        if n in repo:
         mapping[n] = ()
 
     # remove entries about unknown nodes