CI: RSpec tests in stable branch run against HGitaly default branch
This is despite an explicit check of current commit branch and I think it is due to the lack of
CI_COMMIT_HG_BRANCH in the pipeline environment variables for MR pipelines.
This is not Heptapod specific, as
CI_COMMIT_BRANCH is also missing, but we should take a look at the rationale for the change on GitLab side.
(not touching this right before a release, given the big feedback time)