Move `mirror_state.json` under `meta` repository

[ikhoon]

mirror_state.json is automatically created for Git mirror repositories and updated when new commits are found in the upstream repository while running mirroring tasks.

There is no problem when mirroring the entire repository, but if a specific path of a repository is mirrored, mirror_state.json can be updated by the changes not included in the path. As there is no change in the corresponding path, the new commit history would be unnecessary from users' point of view.

I propose to move mirror_state.json under the internal meta repository to keep users' repository clean when there are no changes in the actual content.