Show additional status information during repository import

Summary:
Ref T2350. Fixes T2231.

  - Adds log flags around discovery.
  - Adds message flags for "needs update". This is basically an out-of-band hint to the daemons that a repository should be pulled sooner than normal. We set the flag when users push a revision, and expose a Conduit method that `arc land` will be able to use.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2350, T2231

Differential Revision: https://secure.phabricator.com/D7467