Merge branch 'hn/reftable' into pu

A new refs backend "reftable" to replace the traditional
combination of packed-refs files and one-file-per-ref loose refs
has been implemented and integrated for improved performance and
atomicity.

At v9
cf. <[email protected]>

* hn/reftable:
  Reftable support for git-core
  Add reftable library
  reftable: clarify how empty tables should be written
  reftable: define version 2 of the spec to accomodate SHA256
  reftable: file format documentation
  Add .gitattributes for the reftable/ directory
  refs: document how ref_iterator_advance_fn should handle symrefs
  create .git/refs in files-backend.c
  Iterate over the "refs/" namespace in for_each_[raw]ref
  refs.h: clarify reflog iteration order

Author: gitster

Documentation/Makefile

@@ -93,6 +93,7 @@ TECH_DOCS += technical/protocol-capabilities
 TECH_DOCS += technical/protocol-common
 TECH_DOCS += technical/protocol-v2
 TECH_DOCS += technical/racy-git
+TECH_DOCS += technical/reftable
 TECH_DOCS += technical/send-pack-pipeline
 TECH_DOCS += technical/shallow
 TECH_DOCS += technical/signature-format