Clean up Diffusion tag list query a bit

Summary:

• Use DiffusionCommitQuery.
• Use denormalized summary.
• Use stronger "Importing" test.

Test Plan: Viewed /diffusion/X/ for repos with tags. Saw tags; saw importing commits marked as "Importing" instead of "Unknown".

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7452