citemplate: extend merge-request rules
This is a copy-paste of the merge-pipeline template rules [1] with a small addition.
The template on its own only checks if
$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH and doesn't take into
account that there might be stable branches which you want the
pipeline to still run.
Thus we add a check with GST_UPSTREAM_BRANCH that we have
already been using for similar purposes.
Edited by Jordan Petridіs