Merge branch 'ps/odb-make-creation-pluggable' into seen

The creation of the on-disk data structures for the object database has been made pluggable, allowing future backends to customize their setup. As part of this, the initialization of the object database has been deferred, and the loading of the loose-object map has been detangled from repository initialization. * ps/odb-make-creation-pluggable: odb: make creation of on-disk structures pluggable odb/source: introduce function to map source type to name setup: defer object database creation setup: detangle loading of loose object maps loose: load loose object map for the correct source