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.

contrib/heptapod-ci.yml

@@ -11,7 +11,7 b' variables:'
     TEST_HGTESTS_ALLOW_NETIO: "0"
 
 .all_template: &all
-  when: always
+  when: on_success
 
 .runtests_template: &runtests
     <<: *all
@@ -49,11 +49,11 b' rust-cargo-test-py3:'
 
 phabricator-refresh:
     stage: phabricator
-    rules:
-      - if: '"$PHABRICATOR_TOKEN" != "NO-PHAB"'
-        when: on_success
-      - if: '"$PHABRICATOR_TOKEN" == "NO-PHAB"'
-        when: never
+    except:
+      refs:
+        - merge_requests
+      variables:
+        - $PHABRICATOR_TOKEN == "NO-PHAB"
     variables:
       DEFAULT_COMMENT: ":white_check_mark: refresh by Heptapod after a successful CI run (:octopus: :green_heart:)"
       STABLE_COMMENT: ":white_check_mark: refresh by Heptapod after a successful CI run (:octopus: :green_heart:)\n⚠ This patch is intended for stable ⚠\n{image https://media.giphy.com/media/nYI8SmmChYXK0/source.gif}"