Merge branch 'ce-avoid_es_loading_commits' into 'master'

Backport of avoid_es_loading_commits

See merge request gitlab-org/gitlab-ce!26301

lib/gitlab/git/commit.rb

@@ -184,11 +184,12 @@ module Gitlab
         end
       end
 
-      def initialize(repository, raw_commit, head = nil)
+      def initialize(repository, raw_commit, head = nil, lazy_load_parents: false)
         raise "Nil as raw commit passed" unless raw_commit
 
         @repository = repository
         @head = head
+        @lazy_load_parents = lazy_load_parents
 
         init_commit(raw_commit)
       end
@@ -225,6 +226,12 @@ module Gitlab
         author_name != committer_name || author_email != committer_email
       end
 
+      def parent_ids
+        return @parent_ids unless @lazy_load_parents
+
+        @parent_ids ||= @repository.commit(id).parent_ids
+      end
+
       def parent_id
         parent_ids.first
       end