In https://gitlab.com/gitlab-org/gitlab-ce/issues/66200, we saw that
many clients accidentally request diverging counts for all branches only
because there are no stale/active branches in the project. This has been
fixed on the frontend in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32496.

To prevent this endpoint from calling Gitaly too many times, we require
that branch names be specified if the total number of branches exceeds
the limit (20).