Only delete instances when merging when there is an exact match.
Summary: Previously, we deleted if the heads matched, which meant that we effectively were picking an arbitrary instance if there were incompatible instances. The new behavior makes more sense, although without incoherent instances you are unlikely to be able to do anything useful with the instances. Signed-off-by: Edward Z. Yang <firstname.lastname@example.org> Test Plan: validate Reviewers: austin, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2596
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