Fix GraphQL tree query erroring when using tags

lib/gitlab/git/rugged_impl/repository.rb

@@ -70,7 +70,7 @@ module Gitlab
 
         # Lookup for rugged object by oid or ref name
         def lookup(oid_or_ref_name)
-          rugged.rev_parse(oid_or_ref_name)
+          rev_parse_target(oid_or_ref_name)
         end
       end
     end

spec/models/repository_spec.rb

@@ -2381,7 +2381,7 @@ describe Repository do
     end
   end
 
-  describe '#tree' do
+  shared_examples '#tree' do
     context 'using a non-existing repository' do
       before do
         allow(repository).to receive(:head_commit).and_return(nil)
@@ -2399,10 +2399,17 @@ describe Repository do
     context 'using an existing repository' do
       it 'returns a Tree' do
         expect(repository.tree(:head)).to be_an_instance_of(Tree)
+        expect(repository.tree('v1.1.1')).to be_an_instance_of(Tree)
       end
     end
   end
 
+  it_behaves_like '#tree'
+
+  describe '#tree? with Rugged enabled', :enable_rugged do
+    it_behaves_like '#tree'
+  end
+
   describe '#size' do
     context 'with a non-existing repository' do
       it 'returns 0' do