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 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

Test Plan: Looked at Git, Mercurial and Subversion repositories that were empty and non-empty. Looked at branches/heads. Tried to look at invalid commits. Looked at tags. All of this still works, and some behaviors are a bit better than they used to be.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1493

Differential Revision: https://secure.phabricator.com/D7484