batches: add fork indicators in the UI
Created by: LawnGnome
This deviates a bit from the design in #17879 (closed): that mockup conflates the base repo and branch, and is less clear about what is going to be merged into what. Here's how this PR looks (note the second row, in particular):
Requesting a review from @rrhyne in light of the above.
Part three of #17879 (closed).