Merge branch 'hn/reftable'

The "reftable" backend for the refs API, without integrating into the refs subsystem, has been added. * hn/reftable: Add "test-tool dump-reftable" command. reftable: add dump utility reftable: implement stack, a mutable database of reftable files. reftable: implement refname validation reftable: add merged table view reftable: add a heap-based priority queue for reftable records reftable: reftable file level tests reftable: read reftable files reftable: generic interface to tables reftable: write reftable files reftable: a generic binary tree implementation reftable: reading/writing blocks Provide zlib's uncompress2 from compat/zlib-compat.c reftable: (de)serialization for the polymorphic record type. reftable: add blocksource, an abstraction for random access reads reftable: utility functions reftable: add error related functionality reftable: add LICENSE hash.h: provide constants for the hash IDs