Merge branch 'kn/ref-location'

Allow the directory in which reference backends store their data to be specified. * kn/ref-location: refs: add GIT_REFERENCE_BACKEND to specify reference backend refs: allow reference location in refstorage config refs: receive and use the reference storage payload refs: move out stub modification to generic layer refs: extract out `refs_create_refdir_stubs()` setup: don't modify repo in `create_reference_database()`