Fix a typo in Gitlab::Git::Repository
The typo was discovered from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6445 and this merge request ported https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6445/diffs?commit_id=bc523d96d3e3836ccfc03d7aeee15e8c7adbde5c to CE.