Merge branch 'kn/ref-location' into jch

A mechanism to specify what reference backend to use and store
references in which directory is introduced, which would likely to
be useful during ref migration.

Comments?

* kn/ref-location:
  refs: add GIT_REF_URI to specify reference backend and directory
  refs: support obtaining ref_store for given dir

diff --cc refs.c
Simple merge

diff --cc t/meson.build
Simple merge