Stable iterators for DiffSets.
Add tests around use of CopyOnWriterAfterIterationHashSet
Revert fix to MergeNodeAction now that the underlying cause has been fixed in kernel
Merge pull request #1693 from boggle/2.0.1-cow-diff-sets

[AFTER GA] 2.0.1 cow diff sets
Edit files with Notepad if Desktop.edit fails

The previous code, with its clever use of rundll32 url.dll,FileProtocolHandler is not robust across various versions of Windows.