lfs mirror extend can only merge a non-mirrored layout file, so that when we try to merge multiple mirrors with "-N" option, it needs to call mirror extend multiple times.

As LU-18746 allows to create stale mirror, we could take a further step to extend mirrored layout, and this implies the to-be-merged mirrors would be marked stale, and they can be resyn-ed later with "lfs mirror resync".