Introduce the following variables

- CI_MERGE_REQUEST_ID
- CI_MERGE_REQUEST_IID
- CI_MERGE_REQUEST_REF_PATH
- CI_MERGE_REQUEST_PROJECT_ID
- CI_MERGE_REQUEST_PROJECT_PATH
- CI_MERGE_REQUEST_PROJECT_URL
- CI_MERGE_REQUEST_TARGET_BRANCH_NAME
- CI_MERGE_REQUEST_SOURCE_PROJECT_ID
- CI_MERGE_REQUEST_SOURCE_PROJECT_PATH
- CI_MERGE_REQUEST_SOURCE_PROJECT_URL
- CI_MERGE_REQUEST_SOURCE_BRANCH_NAME