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 separate query paths, "stablecommitname" and "expandshortcommit". These are pretty much doing the same thing -- taking some ref like "master" or "default" or a tag name or part of a commit name, and turning it into a full commit name. Merge them into a single "resolverefs" method.
This simplifies the code a fair bit, and gives us better error messages. They still aren't great, but they're like this now:
Ref "7498aec194ecf2d333e0e2baddd9d5cdf922d7f1" is ambiguous or does not exist.
...instead of just:
ERR-INVALID-COMMIT