R1:7f43cde82dd1

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:
{F384335}

{F384336}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7100

Differential Revision: https…