Fix 'New merge request' button for users who don't have push access to canonical project
Fixes bug introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7342.
For some reason this bug in combination with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12499 started showing this error: https://gitlab.com/gitlab-org/gitlab-ce/issues/34511.