Bring back Rugged implementation of CommitIsAncestor

This brings back some of the changes in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20095.

For users using Gitaly on top of NFS, accessing the Git data directly
via Rugged is more performant than Gitaly. This merge request introduces
the feature flag `rugged_commit_is_ancestor` to activate the Rugged
path.

Part of four Rugged changes identified in
https://gitlab.com/gitlab-org/gitlab-ce/issues/57317.

changelogs/unreleased/sh-rugged-commit-is-ancestor.yml

+---
+title: Bring back Rugged implementation of CommitIsAncestor
+merge_request: 25702
+author:
+type: other

lib/gitlab/git/repository.rb

@@ -11,7 +11,7 @@ module Gitlab
       include Gitlab::Git::WrapsGitalyErrors
       include Gitlab::EncodingHelper
       include Gitlab::Utils::StrongMemoize
-      include Gitlab::Git::RuggedImpl::Repository
+      prepend Gitlab::Git::RuggedImpl::Repository
 
       SEARCH_CONTEXT_LINES = 3
       REV_LIST_COMMIT_LIMIT = 2_000

lib/gitlab/git/rugged_impl/repository.rb

@@ -10,7 +10,9 @@ module Gitlab
   module Git
     module RuggedImpl
       module Repository
-        FEATURE_FLAGS = %i(rugged_find_commit).freeze
+        extend ::Gitlab::Utils::Override
+
+        FEATURE_FLAGS = %i(rugged_find_commit rugged_tree_entries rugged_tree_entry rugged_commit_is_ancestor).freeze
 
         def alternate_object_directories
           relative_object_directories.map { |d| File.join(path, d) }
@@ -41,6 +43,29 @@ module Gitlab
           obj = rugged.rev_parse(revspec)
           Ref.dereference_object(obj)
         end
+
+        override :ancestor?
+        def ancestor?(from, to)
+          if Feature.enabled?(:rugged_commit_is_ancestor)
+            rugged_is_ancestor?(from, to)
+          else
+            super
+          end
+        end
+
+        def rugged_is_ancestor?(ancestor_id, descendant_id)
+          return false if ancestor_id.nil? || descendant_id.nil?
+
+          rugged_merge_base(ancestor_id, descendant_id) == ancestor_id
+        rescue Rugged::OdbError
+          false
+        end
+
+        def rugged_merge_base(from, to)
+          rugged.merge_base(from, to)
+        rescue Rugged::ReferenceError
+          nil
+        end
       end
     end
   end

spec/models/repository_spec.rb

@@ -2237,7 +2237,7 @@ describe Repository do
     rugged.references.create("refs/remotes/#{remote_name}/#{branch_name}", target.id)
   end
 
-  describe '#ancestor?' do
+  shared_examples '#ancestor?' do
     let(:commit) { repository.commit }
     let(:ancestor) { commit.parents.first }
 
@@ -2261,6 +2261,20 @@ describe Repository do
     end
   end
 
+  describe '#ancestor? with Gitaly enabled' do
+    it_behaves_like "#ancestor?"
+  end
+
+  describe '#ancestor? with Rugged enabled', :enable_rugged do
+    it 'calls out to the Rugged implementation' do
+      allow_any_instance_of(Rugged).to receive(:merge_base).with(repository.commit.id, Gitlab::Git::BLANK_SHA).and_call_original
+
+      repository.ancestor?(repository.commit.id, Gitlab::Git::BLANK_SHA)
+    end
+
+    it_behaves_like '#ancestor?'
+  end
+
   describe '#archive_metadata' do
     let(:ref) { 'master' }
     let(:storage_path) { '/tmp' }