Fix Bitbucket Server import only including first 25 pull requests
The change to paginate repos in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22825 caused the paginator to stop after 25 pull requests because the limit was set to 25 if none was defined. To fix this, we should only stop if the limit has actually been set and use the limit parameter to determine the maximum number of items to process per page.