git/objectpool: Refuse creating pools when empty target dir exists

The git-clone(1) command will happily clone into a target directory, but only in the case it is an empty directory. This behaviour is something we have carried over when creating object pools. It is arguably not the right thing to do though: when we're asked to create something that exists already in some kind of form then we really ought to refuse the operation and return an error code.

Tighten our APIs to always refuse creating of object pools when the target directory exists.

Changelog: changed