Empty subproject commits can't assimilate -> split roundtrip

[laughinghan]

To repro:

1. Add an "upstream" empty commit to the subproject branch
2. Assimilate subproject branch
3. Split out the assimilated commits

The empty commit vanishes from the round-trip split out commits. Subsequent commits will all have different hashes. Assimilate needs to detect these and complain. With some statefulness, like a .gitsubhistory folder or something, we could potentially toggle a file in there, though that seems kinda hacky.