Branches index view is performing n + 1 requests

The branches page, like https://gitlab.com/gitlab-org/gitlab-ce/branches, is hugely expensive to calculate and render.

Right now there are 2 N + 1 gitaly endpoints in one controller. Futher, its not thought through to have both active and stale as sections. Just to detect an active branch we might loop through all branches but master and walk the tree for 1000 commits each!

The branches index test does over 100 Gitaly request, which is hurting the page performance and Gitaly.

@jramsay when can this be scheduled? Could you squeeze it in V11.2?

/cc @DouweM