ci: do not trigger phabricator for merge-request
The fast the phabricator steps has a `rules` entry makes it selected for the
special `merge_requests` pipelines. The other ones behave as default and are not
selected tot the mrege_request pipelines.
This result in a second pipeline to be created, with only the phabricator
pipeline in it. Which usually succeed fast (since there is nothing to do).
This is harmful as this create a false sense of "the series is passing" and
Gitlab will use this simplistic pipeline for validation.
By explicitly preventing the pipeline to be created in the merge-request case,
we prevent this situation to happens
Note that the job will be dropped (alonside phabricator) in the next two weeks
anyway.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).