Feature Request. Instead of (or perhaps in addition to) setting
`annex.diskreserve`, I'd like to be able to tell git-annex "use up to 2TB
of disk space".

> This would need changes for git-annex to keep a running total of the
> space it's using, which becomes a little hairy in light of concurrency,
> possibly manual changes to the object store, etc. --[[Joey]]
