stop ProcessCommitWorker from processing MR merge commit

When a merge request is merged, it creates a commit with the
description of the MR, which may contain references and issue
closing references. As this will be handled in the PostMergeService
anyways, let's ignore merge commit generated from a MR.

app/models/commit.rb

@@ -417,6 +417,10 @@ class Commit
     !!(title =~ WIP_REGEX)
   end
 
+  def merged_merge_request?(user)
+    !!merged_merge_request(user)
+  end
+
   private
 
   def commit_reference(from, referable_commit_id, full: false)
@@ -445,10 +449,6 @@ class Commit
     changes
   end
 
-  def merged_merge_request?(user)
-    !!merged_merge_request(user)
-  end
-
   def merged_merge_request_no_cache(user)
     MergeRequestsFinder.new(user, project_id: project.id).find_by(merge_commit_sha: id) if merge_commit?
   end

app/workers/process_commit_worker.rb

@@ -23,27 +23,25 @@ class ProcessCommitWorker
     return unless user
 
     commit = build_commit(project, commit_hash)
-
     author = commit.author || user
 
-    process_commit_message(project, commit, user, author, default)
+    # this is a GitLab generated commit message, ignore it.
+    return if commit.merged_merge_request?(user)
 
+    process_commit_message(project, commit, user, author, default)
     update_issue_metrics(commit, author)
   end
 
   def process_commit_message(project, commit, user, author, default = false)
     closed_issues = default ? commit.closes_issues(user) : []
 
-    unless closed_issues.empty?
-      close_issues(project, user, author, commit, closed_issues)
-    end
-
+    close_issues(project, user, author, commit, closed_issues) if closed_issues.any?
     commit.create_cross_references!(author, closed_issues)
   end
 
   def close_issues(project, user, author, commit, issues)
     # We don't want to run permission related queries for every single issue,
-    # therefor we use IssueCollection here and skip the authorization check in
+    # therefore we use IssueCollection here and skip the authorization check in
     # Issues::CloseService#execute.
     IssueCollection.new(issues).updatable_by_user(user).each do |issue|
       Issues::CloseService.new(project, author)

spec/workers/process_commit_worker_spec.rb

@@ -20,6 +20,32 @@ describe ProcessCommitWorker do
       worker.perform(project.id, -1, commit.to_hash)
     end
 
+    context 'when commit is a merge request merge commit' do
+      let(:merge_request) do
+        create(:merge_request,
+               description: "Closes #{issue.to_reference}",
+               source_branch: 'feature-merged',
+               target_branch: 'master',
+               source_project: project)
+      end
+
+      let(:commit) do
+        project.repository.create_branch('feature-merged', 'feature')
+
+        sha = project.repository.merge(user,
+                                       merge_request.diff_head_sha,
+                                       merge_request,
+                                       "Closes #{issue.to_reference}")
+        project.repository.commit(sha)
+      end
+
+      it 'does not process the commit' do
+        expect(worker).not_to receive(:close_issues)
+
+        worker.perform(project.id, user.id, commit.to_hash)
+      end
+    end
+
     it 'processes the commit message' do
       expect(worker).to receive(:process_commit_message).and_call_original