Summary:
Keepset computation algorithm added tuples to the keepset that are the results
of diffs between two manifests. Now it has been fixed and only new node from
the tuple is added to the keepset.
Test Plan:
* Run hg gc on www, fbsource
* Ensure the unit tests pass
Reviewers: simonfar, durham, #fbhgext, ryanmce
Reviewed By: #fbhgext, ryanmce
Subscribers: ryanmce
Differential Revision: https://phab.mercurial-scm.org/D519
