This adds the reftable library, and hooks it up as a ref backend.

Feedback wanted:

 * spots marked with XXX in the Git source code.
 * how to write a unittest script? From the command-line, it works; inside
   the test framework, it says:

fatal: could not open '/usr/local/google/home/hanwen/vc/git/t/trash directory.t0031-reftable/.git/refs/heads' for writing: Is a directory

ie. the files backend already initialized.

 * what is a good test strategy? Could we have a CI flavor where we flip the
   default to reftable, and see how it fares?

v6

 * implement reflog expiry.

v7

 * support SHA256 as version 2 of the format.

Han-Wen Nienhuys (5):
  refs.h: clarify reflog iteration order
  create .git/refs in files-backend.c
  refs: document how ref_iterator_advance_fn should handle symrefs
  Add reftable library
  Reftable support for git-core

 .../technical/repository-version.txt          |    7 +
 Makefile                                      |   24 +-
 builtin/clone.c                               |    4 +-
 builtin/init-db.c                             |   57 +-
 cache.h                                       |    4 +-
 refs.c                                        |   20 +-
 refs.h                                        |    8 +-
 refs/files-backend.c                          |    6 +
 refs/refs-internal.h                          |    6 +
 refs/reftable-backend.c                       |  971 +++++++++++++++
 reftable/LICENSE                              |   31 +
 reftable/README.md                            |   19 +
 reftable/VERSION                              |   11 +
 reftable/basics.c                             |  196 +++
 reftable/basics.h                             |   37 +
 reftable/block.c                              |  413 ++++++
 reftable/block.h                              |   71 ++
 reftable/blocksource.h                        |   20 +
 reftable/bytes.c                              |    0
 reftable/config.h                             |    1 +
 reftable/constants.h                          |   25 +
 reftable/dump.c                               |   97 ++
 reftable/file.c                               |   97 ++
 reftable/iter.c                               |  229 ++++
 reftable/iter.h                               |   56 +
 reftable/merged.c                             |  290 +++++
 reftable/merged.h                             |   34 +
 reftable/pq.c                                 |  114 ++
 reftable/pq.h                                 |   34 +
 reftable/reader.c                             |  718 +++++++++++
 reftable/reader.h                             |   52 +
 reftable/record.c                             | 1107 +++++++++++++++++
 reftable/record.h                             |   79 ++
 reftable/reftable.h                           |  405 ++++++
 reftable/slice.c                              |  199 +++
 reftable/slice.h                              |   39 +
 reftable/stack.c                              |  984 +++++++++++++++
 reftable/stack.h                              |   40 +
 reftable/system.h                             |   59 +
 reftable/tree.c                               |   66 +
 reftable/tree.h                               |   24 +
 reftable/writer.c                             |  630 ++++++++++
 reftable/writer.h                             |   46 +
 reftable/zlib-compat.c                        |   92 ++
 repository.c                                  |    2 +
 repository.h                                  |    3 +
 setup.c                                       |   12 +-
 47 files changed, 7407 insertions(+), 32 deletions(-)
 create mode 100644 refs/reftable-backend.c
 create mode 100644 reftable/LICENSE
 create mode 100644 reftable/README.md
 create mode 100644 reftable/VERSION
 create mode 100644 reftable/basics.c
 create mode 100644 reftable/basics.h
 create mode 100644 reftable/block.c
 create mode 100644 reftable/block.h
 create mode 100644 reftable/blocksource.h
 create mode 100644 reftable/bytes.c
 create mode 100644 reftable/config.h
 create mode 100644 reftable/constants.h
 create mode 100644 reftable/dump.c
 create mode 100644 reftable/file.c
 create mode 100644 reftable/iter.c
 create mode 100644 reftable/iter.h
 create mode 100644 reftable/merged.c
 create mode 100644 reftable/merged.h
 create mode 100644 reftable/pq.c
 create mode 100644 reftable/pq.h
 create mode 100644 reftable/reader.c
 create mode 100644 reftable/reader.h
 create mode 100644 reftable/record.c
 create mode 100644 reftable/record.h
 create mode 100644 reftable/reftable.h
 create mode 100644 reftable/slice.c
 create mode 100644 reftable/slice.h
 create mode 100644 reftable/stack.c
 create mode 100644 reftable/stack.h
 create mode 100644 reftable/system.h
 create mode 100644 reftable/tree.c
 create mode 100644 reftable/tree.h
 create mode 100644 reftable/writer.c
 create mode 100644 reftable/writer.h
 create mode 100644 reftable/zlib-compat.c

base-commit: e68e29171cc2d6968902e0654b5687fbe1ccb903

Submitted-As: https://lore.kernel.org/git/pull.539.v6.git.1582015420.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.539.git.1579808479.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.539.v2.git.1580134944.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.539.v3.git.1580848060.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.539.v4.git.1581029756.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.539.v5.git.1581344060.gitgitgadget@gmail.com