RefService: always ensure all GitLab state files

As part of heptapod#431 (closed), we will need to honour read requests for special refs. We will have several mechanisms in place to ensure the presence of the various state files for refs, notably as part of Mercurial transactions.

Still, in case of a repo that has been inactive since those mechanisms have been in place, we will need to call the "ensure" function from HGitaly reads. This doesn't create a large performance cost, it's just treating the error return if a file is missing.

For consistency and simplicity, it's also time to replace the fallback code in FindAllBranchNAmes etc by the same ensuring logic.