Add merge request environment variables

Description

For example, I want to check syntax only on files changed by the merge request.

Something like this in .gitlab-ci.yml:

syntax-check:
  script:
    - git checkout $CI_MERGE_TARGET_REF_NAME
    - git merge $CI_COMMIT_REF_NAME
    - for file in $(git diff --diff-filter=d HEAD^  --name-only $CI_MERGE_TARGET_REF_NAME..HEAD); do echo "Checking $file"; done

Documentation blurb

To add in https://docs.gitlab.com/ce/ci/variables/README.html#predefined-variables-environment-variables

CI_MERGE_TARGET_REF_NAME The target branch of the merge request.

Related to #4176 (moved)

I think this is a subset of https://gitlab.com/gitlab-org/gitlab-ce/issues/23902. ;)

closed