Merge Request base branch variable

Background:

We need merge request base branch to check the diff between the base branch and the last merge request commit.

What questions are you trying to answer?

Would it be possible to add a variable equivalent to TRAVIS_BRANCH (see doc here)? Does perhaps someone have a solution to set it in the .gitlab-ci.yml temporarily?

Same question goes for a CI_MERGE_REQUEST the merge request number if it is a merge request.

https://docs.travis-ci.com/user/environment-variables/

Thanks folks,