campaigns: initial UI state after applying a campaign can be wonky

Created by: LawnGnome

When applying a campaign, it is unfortunately common to see a set of errors in the UI while the initial resolve and sync occur, seemingly around diff and diffstat calculation.

Our current best guess is that this is the mechanism:

1. Campaign imports changesets that use branches that have just been pushed.
2. Reconciler attempts to calculate the diff, but gitserver hasn't yet pulled the new branch.
3. Diffstats don't appear, red boxes appear in the UI, expanding a changeset to view a diff doesn't result in the expected diff either.