Remove redundant Repository#exists? checks

Removing checks of Repository#exists? before using Repository#root_ref,
as #root_ref returns nil when a repository does not exist.

app/helpers/search_helper.rb

@@ -143,7 +143,7 @@ module SearchHelper
 
   # Autocomplete results for the current project, if it's defined
   def project_autocomplete
-    if @project && @project.repository.exists? && @project.repository.root_ref
+    if @project && @project.repository.root_ref
       ref = @ref || @project.repository.root_ref
 
       [

app/models/project.rb

@@ -1514,7 +1514,7 @@ class Project < ApplicationRecord
   end
 
   def default_branch
-    @default_branch ||= repository.root_ref if repository.exists?
+    @default_branch ||= repository.root_ref
   end
 
   def reload_default_branch

spec/models/repository_spec.rb

@@ -1331,6 +1331,13 @@ describe Repository do
       repository.root_ref
     end
 
+    it 'returns nil if the repository does not exist' do
+      repository = create(:project).repository
+
+      expect(repository).not_to be_exists
+      expect(repository.root_ref).to be_nil
+    end
+
     it_behaves_like 'asymmetric cached method', :root_ref
   end