Thinning a repository

Hi everyone,

Is there a quick way to reproduce the effect of a shallow clone on a
local repository that doesn't involve filter-branch and/or re-clone?

My motivation is to reduce the local size of repositories I'm only
following, by trimming the history without prejudice to a [N] set of
last commits. It feels stupid that the quickest way I'm aware of right
now to achieve this is to "git clone --depth N ..." again. filter-branch
is ridiculously slow, as it iterates through history.

I've tried using graft points, but the combination of:

echo [sha] > .git/info/grafts
git reflog expire --expire=0 --all
git repack -Ad

doesn't really save any space and/or reduce the object count as I would
expect. It means there's probably still reachable?

I'd really love to have a 'git thin [depth]' subcommand to perform the
above however. I don't really want to have to iterate through refs just
to check if they are still reachable within [n] commits just to delete them.

Thanks for any pointer.