odb: make creation of on-disk structures pluggable

odb: make creation of on-disk structures pluggable

When creating a new "files" object database source we have to create a
couple of directories. These directories are of course specific to this
particular backend, and a different backend may require a setup that is
completely different.

Make the creation of on-disk structures pluggable to accommodate for
this.

Note that there is one exception though: the "objects" directory must
exist in a repository regardless of which backend is in use. If it
doesn't exist then the repository is not treated as a Git repository at
all. Consequently, we create this directory regardless of the backend.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>