Summary:
There's a lot of code duplication in the differ. Reduce by factoring a duplicated code path into `updateMatchedPairSubtrees`.
This handles cases of updating trees with flattening or unflattening.
Changelog: [Internal]
Reviewed By: sammy-SC
Differential Revision: D28003339
fbshipit-source-id: cbbf890ba447b29d79aedea374b173de40e71334