Fix using ancestor? rather than is_ancestor?

app/models/repository.rb

@@ -1018,7 +1018,7 @@ class Repository
     upstream_commit = commit("refs/remotes/#{MIRROR_REMOTE}/#{branch_name}")
 
     if upstream_commit
-      is_ancestor?(branch_commit.id, upstream_commit.id)
+      ancestor?(branch_commit.id, upstream_commit.id)
     else
       false
     end

ee/app/models/ee/merge_request.rb

@@ -14,7 +14,7 @@ module EE
     end
 
     def ff_merge_possible?
-      project.repository.is_ancestor?(target_branch_sha, diff_head_sha)
+      project.repository.ancestor?(target_branch_sha, diff_head_sha)
     end
 
     def should_be_rebased?