Limit diverging commit counts requests

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).