Add tests and one new feature to SplitConflictMerger
This is split from If4d4845 (and because of this tagged as being part
of T195724). The two patches can be merged in any order, because the
edits to SplitConflictMerger are duplicated and do not conflict. I made
this a separate, independent patch to speed review up.
Bug: T195724
Change-Id: If855e60d3d40a7939d3ff11ebdd09f0ac82ac8c2