Add a "refs" table to Diffusion

Summary:
Ref T7100. When a user navigates to a branch like "default" which is ambiguous:

• don't fatal;
• choose one alternative to resolve it to (currently more or less at random);
• sometimes show what we did in the UI.

Also, add a new table to show the alternatives.

This will get refined in followup changes.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7100

Differential Revision: https://secure.phabricator.com/D12547