allow diff.branch between unrelated branches

[mitchellwrosen]

Overview

This PR lifts the restriction in diff.branch that disallows diffing branches that don't share a history. We had always planned to handle this case by effectively setting the LCA to the first argument, however the restriction was originally added for performance reasons, as the implementation in this PR isn't efficient (it just sets the causal hash of the LCA, and then proceeds with the rest of the algorithm, which will end up doing unnecessary work in several places). Nonetheless, it now seems like a better idea to just do the inefficient thing and follow up with a more efficient thing soon.

Test coverage

Transcripts and manual testing

Loose ends

More efficient implementation in progress