ci: Avoid duplicate pipelines vol 2

This is a followup to GNOME/mutter!1578

This commit does a couple of things to avoid creating multiple
pipelines per commit.

First, it avoid catch all `when: manual` rules, which might
end up matching custom variables set which might potentially
not be handled.

Secondly it reworks the `workflow:rules:` and the pipeline guard
rules to avoid duplicate pipelines as the gitlab documentation
suggests.

Last, it switches from yaml anchors to the new `reference` gitlab
keyword which is more flexible.

https://docs.gitlab.com/ee/ci/jobs/job_control.html#avoid-duplicate-pipelines

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2534>

.gitlab-ci.yml

@@ -146,17 +146,31 @@ default:
 
 workflow:
   rules:
-    - if: '$CI_MERGE_REQUEST_IID'
-    - if: '$CI_COMMIT_TAG'
+    # Allow to switch from branch pipelines to MR pipelines seamlessly
+    # https://docs.gitlab.com/ee/ci/jobs/job_control.html#avoid-duplicate-pipelines
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS && $CI_PIPELINE_SOURCE == "push"
+      when: never
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    # Don't trigger a branch pipeline if there is an open MR
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+      when: never
     - if: '$CI_COMMIT_BRANCH'
+    - if: '$CI_COMMIT_TAG'
 
-.pipline-guard: &pipline-guard
+.pipeline-guard:
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
     - if: '$CI_COMMIT_TAG'
     - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
     - if: '$CI_COMMIT_BRANCH =~ /^gnome-[0-9-]+$/'
-    - when: 'manual'
+    # Avoid catchall `when: manual` rule which might
+    # cause duplicate pipelines to be triggered.
+    # https://docs.gitlab.com/ee/ci/jobs/job_control.html#avoid-duplicate-pipelines
+    #
+    # Also make it so pipelines without MR need to be started
+    # manually, since their state will most likely be WIP
+    - if: '$CI_COMMIT_BRANCH'
+      when: 'manual'
 
 check-commit-log:
   extends:
@@ -177,7 +191,8 @@ check-commit-log:
       - commit-message-junit-report.xml
     reports:
       junit: commit-message-junit-report.xml
-  <<: *pipline-guard
+  rules:
+    - !reference [.pipeline-guard, rules]
 
 check-merge-request:
   extends:
@@ -198,7 +213,8 @@ check-merge-request:
       - check-merge-request-report.xml
     reports:
       junit: check-merge-request-report.xml
-  <<: *pipline-guard
+  rules:
+    - !reference [.pipeline-guard, rules]
 
 build-fedora-container@x86_64:
   extends:
@@ -447,7 +463,8 @@ test-mutter-coverity:
   rules:
     - if: '$CI_PIPELINE_SOURCE == "schedule" && $MUTTER_SCHEDULED_JOB == "coverity"'
       when: always
-    - when: manual
+    - if: '$CI_COMMIT_BRANCH'
+      when: 'manual'
   extends:
     - .fdo.distribution-image@fedora
     - .mutter.fedora:35@x86_64