Git Sync: allow several repositories to place dashboards in the same folder

Why is this needed: We run Git Sync (Grafana 13.1.0 OSS, pure Git connection to GitLab) with two repositories, both folderless, sharing the same directory layout (one directory per project): • repository A: dashboards edited by teams in the UI (workflows: [write, branch]); • repository B: dashboards generated by CI from Jsonnet, read-only in Grafana (workflows: []), since any UI edit would be overwritten by the next render. Every project that has dashboards in both repositories shows up twice at the root, as two folders with the same title (one marked "Read only"). Users have to look in two places for one project, and folder permissions have to be managed twice. There is no way around it today: • A folder is owned by a single repository: if repository B's _folder.json uses the UID of a folder created by repository A, the sync fails with NewFolderManagedByOtherError ([folders.go#L267]( • A resource's parent folder is always derived from i…