Add CI_COMMIT_FORCE_PUSH variable

What does this MR do and why?

This (untested) change exposes a variable to CI pipelines that allow conditional branching when a force push occurs. The current version of gitlab incorrectly determines the list of files changed when a force push occurs. It would be possible to work around this incorrect handling by giving pipeline authors a variable for branching when a force push has occurred. This MR implements that variable.

• Related to add CI_FORCE_PUSH variable (#409307)
• Related to CI_COMMIT_BEFORE_SHA takes wrong value upon bra... (#357394)

How to set up and validate locally

Configure a .gitlab-ci.yml file with the following:

test:
  stage: test
  script:  
  - echo $CI_COMMIT_FORCE_PUSH

Run the following:

git init
touch foo
git add .
git commit -m "foo"
touch bar
git commit -m "bar"
git push origin main
git reset --hard HEAD~1
touch baz
git add .
git commit -m "baz"
git push origin main --force

Observe that the final push triggers a pipeline wherein CI_COMMIT_FORCE_PUSH is set to "true".

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.