CI variables: CI_COMMIT_BEFORE_SHA is broken

It seems GitLab CI should offer the variable ${CI_COMMIT_BEFORE_SHA}, however it's currently unset.

Interestingly the legacy variable ${CI_BUILD_BEFORE_SHA} works as expected.

Beaviour of ${CI_COMMIT_BEFORE_SHA} should be fixed. Also it should be properly documented (as this variable seems to have been broken at least as often as it was working there seems to be a lot of confusion regarding the actual state).