[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.
add CI variable comparable to
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.