CI_COMMIT_BEFORE_SHA is always zeroes

As provided in documentation, CI_COMMIT_BEFORE_SHA should be present on CI jobs, yet it is always 0000000000000000000000000000000000000000.

Gitlab-ee version: 11.3.4

Gitlab runner version: 11.3.1

Steps to reproduce:

  1. create a branch.
  2. do first commit and push
  3. do second commit and push
  4. look at the Runner trace output

Runner Trace output:

++ export CI_SERVER_NAME=GitLab
++ CI_SERVER_NAME=GitLab
++ export CI_SERVER_VERSION=11.3.4-ee
++ CI_SERVER_VERSION=11.3.4-ee
++ export CI_SERVER_REVISION=14d3a1d
++ CI_SERVER_REVISION=14d3a1d
++ export CI_JOB_NAME=build-service
++ CI_JOB_NAME=build-service
++ export CI_JOB_STAGE=build
++ CI_JOB_STAGE=build
++ export CI_COMMIT_SHA=3225c4284b11a967362842b071ed6fa9c432a048
++ CI_COMMIT_SHA=3225c4284b11a967362842b071ed6fa9c432a048
++ export CI_COMMIT_BEFORE_SHA=0000000000000000000000000000000000000000
++ CI_COMMIT_BEFORE_SHA=0000000000000000000000000000000000000000
Edited by FarhadF