Merge branch 'mh/submodule-hash'

Code and design clean-up for the refs API. * mh/submodule-hash: read_loose_refs(): read refs using resolve_ref_recursively() files_ref_store::submodule: use NULL for the main repository base_ref_store_init(): remove submodule argument refs: push the submodule attribute down refs: store submodule ref stores in a hashmap register_ref_store(): new function refs: remove some unnecessary handling of submodule == "" refs: make some ref_store lookup functions private refs: reorder some function definitions