ci: Check for issue/MR URLs in commit messages

Commit messages should include the full URL to an issue or merge
request to keep associated information easily available in future
log digging.

Jonas came up with a script to enforce that policy for mutter and
gnome-shell. It's an excellent idea, so adopt it here as well.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/merge_requests/52

.gitlab-ci.yml

@@ -1,15 +1,31 @@
 stages:
+ - commit_check
  - source_check
  - build
 
 variables:
  LINT_LOG: "eslint-report.txt"
 
+.only_default: &only_default
+    only:
+        - branches
+        - tags
+        - merge_requests
+
+check_commit_log:
+    image: registry.gitlab.gnome.org/gnome/gjs:fedora.static-analysis
+    stage: commit_check
+    script:
+        - ./.gitlab-ci/check-commit-log.sh
+    only:
+        - merge_requests
+
 eslint:
   image: registry.gitlab.gnome.org/gnome/gjs:fedora.static-analysis
   stage: source_check
   script:
     - sh lint/generate-report.sh -o $LINT_LOG || { cat $LINT_LOG; false; }
+  <<: *only_default
   artifacts:
     paths:
       - ${LINT_LOG}
@@ -23,3 +39,4 @@ build-shell-extensions:
   script:
     - meson _build .
     - ninja -C _build test install
+  <<: *only_default

.gitlab-ci/check-commit-log.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+
+if [ -z "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME" ]; then
+  echo Cannot review non-merge request
+  exit 1
+fi
+
+git fetch $CI_MERGE_REQUEST_PROJECT_URL.git $CI_MERGE_REQUEST_TARGET_BRANCH_NAME
+
+branch_point=$(git merge-base HEAD FETCH_HEAD)
+
+commits=$(git log --format='format:%H' $branch_point..$CI_COMMIT_SHA)
+
+if [ -z "$commits" ]; then
+  echo Commit range empty
+  exit 1
+fi
+
+function commit_message_has_url() {
+  commit=$1
+  commit_message=$(git show -s --format='format:%b' $commit)
+  echo "$commit_message" | grep -qe "\($CI_MERGE_REQUEST_PROJECT_URL/\(issues\|merge_requests\)/[0-9]\+\|https://bugzilla.gnome.org/show_bug.cgi?id=[0-9]\+\)"
+  return $?
+}
+
+for commit in $commits; do
+  if ! commit_message_has_url $commit; then
+    echo "Missing merge request or issue URL on commit $(echo $commit | cut -c -8)"
+    exit 1
+  fi
+done