Convert Gitaly commit parent IDs to array as early as possible

The tracking issue if this causes problems is
https://gitlab.com/gitlab-org/gitaly/issues/1028

lib/gitlab/git/commit.rb

@@ -471,15 +471,6 @@ module Gitlab
 
       private
 
-      def parent_ids=(shas)
-        @parent_ids = case shas
-                      when String
-                        JSON.parse(shas)
-                      else
-                        shas
-                      end
-      end
-
       def init_from_hash(hash)
         raw_commit = hash.symbolize_keys
 
@@ -517,7 +508,7 @@ module Gitlab
         @committed_date = Time.at(commit.committer.date.seconds).utc
         @committer_name = commit.committer.name.dup
         @committer_email = commit.committer.email.dup
-        @parent_ids = commit.parent_ids
+        @parent_ids = Array(commit.parent_ids)
       end
 
       def serialize_keys