RepositoryService.FetchBundle: initial implementation
This is the method used by gitaly-backup
for restoration, full or incremental.
Incrementality is provided simply by the fact that it creates the repository only if needed. In other words, there isn't much to it in the restoration process, all the logic is in the creation of partial bundles (based themselves on the ref files that are parts backups, thanks to CreateBundleFromRefList
).