R1:b49c5e9762db

"Merge Duplicates" in Maniphest

Summary:
Allow duplicate tasks to be selected and merged in Maniphest.

I didn't create a separate transaction type for this because that implies a
bunch of really complicated rules which I don't want to sort out right now
(e.g., do we need to do cycle detection for merges? If so, what do we do when we
detect a cycle?) since I think it's unnecessary to get right for the initial
implementation (my Tasks merge implementation was similar to this and worked
quite well) and…