[RFC PATCH 00/24] Add backup log

"Backup log" is similar to reflog. But instead of keeping track of ref changes, it keeps track of file content changes. These could be from the index (e.g. "git add" replacing something in the index), worktree ("git reset --hard" deleting everything) or in gitdir ("git config --edit", or deleted reflog).

Backup log, when enabled, keeps the backup versions so you can undo if needed. Head for 01/24 to have a better picture what it does, when backups are made... This series adds a new plumbing command 'git backup-log' to manage these backups.

A couple things left to do:

- high level UI design, including maybe extended SHA-1 syntax
- whether "git checkout <paths>" should keep backups. I think doing it
  unconditionally may be too much, but maybe keep backups of files
  with "precious" attribute on
- a UI to edit $GIT_DIR/info/excludes and gitattributes so we can make
  backups of them
- whether we should keep command causing the changes in the backup log
  (e.g. this change is made by git-add, that one git-rebase...).
  Reflog has this. I did not add it because it complicates the parsing
  a bit and not sure if it's worth it.

Nguyễn Thái Ngọc Duy (24):
  doc: introduce new "backup log" concept
  backup-log: add "update" subcommand
  read-cache.c: new flag for add_index_entry() to write to backup log
  add: support backup log
  update-index: support backup log with --keep-backup
  commit: support backup log
  apply: support backup log with --keep-backup
  add--interactive: support backup log
  backup-log.c: add API for walking backup log
  backup-log: add cat command
  backup-log: add diff command
  backup-log: add log command
  backup-log: add prune command
  gc: prune backup logs
  backup-log: keep all blob references around
  sha1-file.c: let index_path() accept NULL istate
  config --edit: support backup log
  refs: keep backup of deleted reflog
  unpack-trees.c: keep backup of ignored files being overwritten
  reset --hard: keep backup of overwritten files
  checkout -f: keep backup of overwritten files
  am: keep backup of overwritten files on --skip or --abort
  rebase: keep backup of overwritten files on --skip or --abort
  FIXME

 .gitignore                         |   1 +
 Documentation/config/core.txt      |   5 +
 Documentation/git-apply.txt        |   3 +
 Documentation/git-backup-log.txt   | 109 ++++++++
 Documentation/git-update-index.txt |   3 +
 Makefile                           |   2 +
 apply.c                            |  38 ++-
 apply.h                            |   1 +
 backup-log.c                       | 388 +++++++++++++++++++++++++++++
 backup-log.h                       |  38 +++
 builtin.h                          |   1 +
 builtin/add.c                      |   5 +
 builtin/am.c                       |   3 +
 builtin/backup-log.c               | 371 +++++++++++++++++++++++++++
 builtin/checkout.c                 |   4 +
 builtin/commit.c                   |  16 +-
 builtin/config.c                   |  27 +-
 builtin/gc.c                       |   3 +
 builtin/pack-objects.c             |   9 +-
 builtin/rebase.c                   |   6 +-
 builtin/repack.c                   |   1 +
 builtin/reset.c                    |   2 +
 builtin/update-index.c             |   7 +
 cache.h                            |   2 +
 command-list.txt                   |   1 +
 git-add--interactive.perl          |  14 +-
 git.c                              |   1 +
 merge-recursive.c                  |   2 +-
 merge.c                            |   2 +
 parse-options.c                    |   2 +-
 reachable.c                        |   3 +
 read-cache.c                       |  49 +++-
 refs/files-backend.c               |  32 +++
 revision.c                         |   3 +
 sha1-file.c                        |   8 +-
 t/t2080-backup-log.sh              | 228 +++++++++++++++++
 unpack-trees.c                     | 143 +++++++++--
 unpack-trees.h                     |   6 +-
 38 files changed, 1488 insertions(+), 51 deletions(-)
 create mode 100644 Documentation/git-backup-log.txt
 create mode 100644 backup-log.c
 create mode 100644 backup-log.h
 create mode 100644 builtin/backup-log.c
 create mode 100755 t/t2080-backup-log.sh