Ensure docs hugo_build CI job uses docs-gitlab-com stable branches

What does this MR do and why?

Previously the docs hugo_build CI job attempted to check out the corresponding stable branch for the docs-gitlab-com repo if it detected a merge request targeted a GitLab stable branch. However, it did not do the same for direct commits to stable branches, which occur after the merge. This caused newly-added rules, such as external image checks, to fail on 18-1-stable-ee.

To fix this problem, also check CI_COMMIT_BRANCH.

References

Relates to gitlab-org/release/tasks#20666 (closed)

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