Introduce the following variables:
 - CI_MERGE_REQUEST_DIFF_ID
 - CI_MERGE_REQUEST_DIFF_BASE_SHA
behind a feature flag, ci_mr_diff_variables.

For testing, remove the context 'when source project does not exist'
because the factory creates the pipeline in the source project. Also
differentiate between detached and merged results pipelines.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/273387