R1:d8bda7c66e0a

Add an "importing" state to repositories and clean up the UI

Summary:
Fixes T3217. Ref T776. Ref T1493. Broadly, this introduces a mechanism which works like this:

- When a repository is created, we set an "importing" flag.
- After discovery completes, we check if a repository has no importing commits. Basically, this is the first time we catch up to HEAD.
- If we're caught up, clear the "importing" flag.

This flag lets us fix some issues:

- T3217. Currently, when you import a new repository…