Major Description
It should be possible to force the resync of a mirror with "lfs mirror resync --only N --force-resync <file>" in order to resync an inconsistent component. Otherwise, "lfs mirror verify -v <file>" can show that two mirrors of a file are inconsistent with each other, but "lfs mirror resync <file>" (with or without the "--only N" option) will not resync the file.
It should be possible to use "lfs setstripe --comp-set --comp-flags=stale --mirror-id=N <file>" to mark at least one component of the mirror stale, without the user having to compute the value for --comp-id=$(((mirror_id << 16) | 1)). It probably makes sense to set the requested flags on all components of that mirror. Alternately, we might consider "lfs mirror stale --mirror-id=N" or "lfs mirror set_flags --comp-flags=stale --mirror-id=N" to do this, since at least it would be more visible within the "lfs mirror" family of commands.
It should be possible to have "lfs mirror verify --stale" (or similar) check the mirrors of a file and mark one of them stale based on best-guess information (e.g. if there are multiple mirrors and only one is different, based on timestamps from the objects, etc).
Running "lfs mirror verify <file>" with a stale component should print something (e.g. "Mirror 2: 0x8f5a8013 (stale)") and at least return a non-zero error code, since it is otherwise misleading that "lfs mirror verify" returns success when in fact the mirrors are not in sync.
Running "lfs mirror verify --only=1,2 <file>" returns an unexpected error:
lfs mirror verify: option '--only=1,2,<file>' unrecognized.
