[CI] Add TRAVIS_COMMIT_RANGE variable

In order to optimize jobs, having the range of affected commits would be really useful. As the CI runner does not know all branches, and master points to the wrong commit, calculating it ourselves is not possible.

Requested change

• add CI variable comparable to TRAVIS_COMMIT_RANGE

The available CI variables do have CI_COMMIT_SHA already, and mention CI_COMMIT_BEFORE_SHA in one of the examples. CI_COMMIT_BEFORE_SHA does not exist though.