Dashboard provisioning: symlink changes in path components cause all dashboards to be deleted & re-created

### What happened? We are using [git-sync](https://github.com/kubernetes/git-sync) with Grafana provisioning to update our dashboards. Whenever we update our dashboard repository, Grafana deletes from its database and re-provisions all the dashboards that were provisioned from this repo. This causes 404s and eg. removes user stars from all affected dashboards. It is reproducible without git-sync (see repro steps below), but I'm describing git-sync here because it's probably a popular way to update provisioned dashboards. The root cause appears to be related to #13360 -- the way git-sync works is it creates git worktrees in `.worktrees/` under its root directory, and a symlink to point to the "current" worktree, also in its root directory. It swaps this symlink target atomically when updates happen, so for example, when we have git-sync watching a repository that contains grafana dashboards, and have set `GITSYNC_ROOT=/dash/root` `GITSYNC_DIR=grafana`: 1. grafana's `file` type dashboard provider is configured to look at `/dash/root/grafana` (see repro steps) 2. when the repo watched by git-sync is updated, git-sync checks out the target ref of the target repository into a directory under `/dash/root/.worktrees/<ref>`, and creates `/dash/root/grafana` symlink that points to `.worktrees/<ref>`. if the symlink existed before, its target directory under `.worktrees/` is deleted by git-sync 3. grafana's dashboard provisioning `updateIntervalSeconds` elapses, it walks`/dash/root/grafana` to discover all json files 4. since grafana resolves the symlink and stores the absolute path each json file was resolved into in `external_id` column of `dashboard_provisioning`, it can no longer find the old files when running dashboard provisioning, and deletes all dashboards. They are then rediscovered in the new directory and re-created. ### What did you expect to happen? at step 4., I expected dashboard provisioning to only update modified dashboards, and not delete/recreate everythi…