Description of the problem, including code/CLI snippet

No merge_base API support, while GitLab v4 has this API.

Seehttps://docs.gitlab.com/ee/api/repositories.html#merge-base

Expected Behavior

Can get merge_base via python-gitlab

Actual Behavior

No API found.

Specifications

• python-gitlab version: 1.15.0
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): 11.4.0