Make CreateGpgSignatureWorker backwards compatible with original method signature

Older versions of GitPushService push a single commit SHA string to the queue,
but Gitaly requires that the parameters sent by CreateGpgSignatureWorker are
an array. It's possible to have old workers using this original signature or
jobs in the retry queue that would fail if CreateGpgSignatureWorker can't
handle the string form.

app/workers/create_gpg_signature_worker.rb

@@ -4,6 +4,10 @@ class CreateGpgSignatureWorker
   include ApplicationWorker
 
   def perform(commit_shas, project_id)
+    # Older versions of GitPushService may push a single commit ID on the stack.
+    # We need this to be backwards compatible.
+    commit_shas = Array(commit_shas)
+
     return if commit_shas.empty?
 
     project = Project.find_by(id: project_id)

spec/workers/create_gpg_signature_worker_spec.rb

@@ -4,10 +4,9 @@ describe CreateGpgSignatureWorker do
   let(:project) { create(:project, :repository) }
   let(:commits) { project.repository.commits('HEAD', limit: 3).commits }
   let(:commit_shas) { commits.map(&:id) }
-
-  context 'when GpgKey is found' do
   let(:gpg_commit) { instance_double(Gitlab::Gpg::Commit) }
 
+  context 'when GpgKey is found' do
     before do
       allow(Project).to receive(:find_by).with(id: project.id).and_return(project)
       allow(project).to receive(:commits_by).with(oids: commit_shas).and_return(commits)
@@ -36,6 +35,16 @@ describe CreateGpgSignatureWorker do
     end
   end
 
+  context 'handles when a string is passed in for the commit SHA' do
+    it 'creates a signature once' do
+      allow(Gitlab::Gpg::Commit).to receive(:new).with(commits.first).and_return(gpg_commit)
+
+      expect(gpg_commit).to receive(:signature).once
+
+      described_class.new.perform(commit_shas.first, project.id)
+    end
+  end
+
   context 'when Commit is not found' do
     let(:nonexisting_commit_sha) { '0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a34' }