Migration to store project full path in repository

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16027#note_53728095 we added a new configuration item to GitLab-managed git repositories, storing the project name as part of the configuration.

In the hashed storage case, this allows us to determine the namespace and project we should import a repository as, as part of a last-ditch "restore from this backup of /var/lib/git/repositories that I have" case.

However, existing installations won't get the new configuration item written unless they are migrated or transferred at some point. To be sure we can rely on this being present, we should consider having a rake task or background migration (latter preferred) to create this configuration item once, for all repositories.

I don't consider it very high-priority - it's only an issue if we're importing hashed storage repos, and when repos are migrated to hashed storage, the configuration will be written. So this only affects repos migrated between %10.0 and %10.3. Still, it's a bit of technical debt that I think is worth clearing up at some stage.