fix: use MR diff SHA in commitlint job
Use the variable $CI_MERGE_REQUEST_DIFF_BASE_SHA instead of $CI_MERGE_REQUEST_TARGET_BRANCH_SHA as it is only available in pipelines for merged results and not available for draft MRs either.
Use the variable $CI_MERGE_REQUEST_DIFF_BASE_SHA instead of $CI_MERGE_REQUEST_TARGET_BRANCH_SHA as it is only available in pipelines for merged results and not available for draft MRs either.