Unison has a neat approach to this problem: References are hashes of the abstract syntax tree, the only way to write a "collision" is to write an identical function--which isn't actually a collision at all.
Good point. I do the same in my own system and use Hashes of the source code, so there are no collisions. Slowly this technique will become mainstream I predict.
At some point we have to communicate which change is correct.
It does not have to be synchronous and it might be via commit message - but still change alone is not enough for conflict resolution.
If you edit word document and someone then changes something there is no commit message but might be comment on document, email or im.