Do not raise error when checking pipeline reference

Return from the `Ci::Pipeline#ref_exists?` in case when there is no
repository present.

This also fixes pipeline page feature specs by changing pipeline
reference instead of stubbing `ref_exist?` method.

app/models/ci/pipeline.rb

@@ -270,6 +270,8 @@ module Ci
 
     def ref_exists?
       project.repository.ref_exists?(git_ref)
+    rescue Gitlab::Git::Repository::NoRepository
+      false
     end
 
     ##

spec/features/projects/pipelines/pipeline_spec.rb

@@ -241,9 +241,12 @@ describe 'Pipeline', :js do
       end
     end
 
-    context 'with deleted branch' do
-      before do
-        allow(pipeline).to receive(:ref_exists?).and_return(false)
+    context 'when pipeline ref does not exist in repository anymore' do
+      let(:pipeline) do
+        create(:ci_empty_pipeline, project: project,
+                                   ref: 'non-existent',
+                                   sha: project.commit.id,
+                                   user: user)
       end
 
       it 'does not render link to the pipeline ref' do

spec/models/ci/pipeline_spec.rb

@@ -780,6 +780,7 @@ describe Ci::Pipeline, :mailer do
   end
 
   describe 'ref_exists?' do
+    context 'when repository exists' do
       using RSpec::Parameterized::TableSyntax
 
       let(:project) { create(:project, :repository) }
@@ -802,6 +803,17 @@ describe Ci::Pipeline, :mailer do
       end
     end
 
+    context 'when repository does not exist' do
+      let(:pipeline) do
+        create(:ci_empty_pipeline, project: project, ref: 'master')
+      end
+
+      it 'always returns false' do
+        expect(pipeline.ref_exists?).to eq false
+      end
+    end
+  end
+
   context 'with non-empty project' do
     let(:project) { create(:project, :repository) }