Draft: Add CI variable CI_COMMIT_SAME_AS_DEFAULT_BRANCH

What does this MR do and why?

This MR adds a new CI variable CI_COMMIT_SAME_AS_DEFAULT_BRANCH, which is used for determining if the pipeline sha is the same as the main branch sha. It returns either true as a string or nothing.

It'll be mainly used for new branches with no new commits.

Related to #15170 (closed)

Screenshots or screen recordings

Example config:

echo:
  script:
    - echo $CI_COMMIT_SAME_AS_DEFAULT_BRANCH

test1:
  script: exit 0
  rules:
    - if: $CI_COMMIT_SAME_AS_DEFAULT_BRANCH

test2:
  script: exit 0
  rules:
    - if: '$CI_COMMIT_SAME_AS_DEFAULT_BRANCH'

test3:
  script: exit 0
  rules:
    - if: '$CI_COMMIT_SAME_AS_DEFAULT_BRANCH == "true"'

test4:
  script: exit 0
  rules:
    - if: '$CI_COMMIT_SAME_AS_DEFAULT_BRANCH != "true"'

When you run on the main branch:

When you run on another branch with a new commit:

MR acceptance checklist

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