Offer slugs of merge request branch names
Original issue: #438861
We have $CI_COMMIT_REF_SLUG for $CI_COMMIT_REF_NAME, why not the same for $CI_MERGE_REQUEST_TARGET_BRANCH_NAME and $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME.
Closes #215273 (closed) and #438861.
Edited by Dmytro Biryukov