R1:4f205308569f

Merge "expandshortcommitquery" and "stablecommitnamequery" into "resolverefs"

Summary:
Ref T1493. Diffusion has some garbagey behavior for things we can't resolve. Common cases are:

- Looking at a branch that doesn't exist.
- Looking at a repository with no branches.
- Looking at a commit that doesn't exist.
- Looking at an empty repository.

In these cases, we generally fatal unhelpfully. I want to untangle this mess.

This doesn't help much, but does clean things up a bit. We currently have two…