[PATCH v6 00/32] Support multiple checkouts

This is basically a reroll from what was parked on 'pu' with two new patches at the end: one to not share info/sparse-checkout across worktrees, and one to allow 'checkout --to` from a bare repository.

I cherry-picked two patches from jk/xstrfmt (on next) because they result in non-trivial conflicts. When this series is merged with jk/xstrfmt, you still get conflicts in environment.c, but you can just pick up my changes and drop Jeff's.

Dennis Kaarsemaker (1):
  checkout: don't require a work tree when checking out into a new one

Jeff King (2):
  setup_git_env: use git_pathdup instead of xmalloc + sprintf
  setup_git_env(): introduce git_path_from_env() helper

Nguyễn Thái Ngọc Duy (29):
  path.c: make get_pathname() return strbuf instead of static buffer
  path.c: make get_pathname() call sites return const char *
  git_snpath(): retire and replace with strbuf_git_path()
  path.c: rename vsnpath() to do_git_path()
  path.c: group git_path(), git_pathdup() and strbuf_git_path() together
  git_path(): be aware of file relocation in $GIT_DIR
  *.sh: respect $GIT_INDEX_FILE
  reflog: avoid constructing .lock path with git_path
  fast-import: use git_path() for accessing .git dir instead of get_git_dir()
  commit: use SEQ_DIR instead of hardcoding "sequencer"
  $GIT_COMMON_DIR: a new environment variable
  git-sh-setup.sh: use rev-parse --git-path to get $GIT_DIR/objects
  *.sh: avoid hardcoding $GIT_DIR/hooks/...
  git-stash: avoid hardcoding $GIT_DIR/logs/....
  setup.c: convert is_git_directory() to use strbuf
  setup.c: detect $GIT_COMMON_DIR in is_git_directory()
  setup.c: convert check_repository_format_gently to use strbuf
  setup.c: detect $GIT_COMMON_DIR check_repository_format_gently()
  setup.c: support multi-checkout repo setup
  wrapper.c: wrapper to open a file, fprintf then close
  use new wrapper write_file() for simple file writing
  checkout: support checking out into a new working directory
  checkout: clean up half-prepared directories in --to mode
  checkout: detach if the branch is already checked out elsewhere
  prune: strategies for linked checkouts
  gc: style change -- no SP before closing bracket
  gc: support prune --repos
  count-objects: report unused files in $GIT_DIR/repos/...
  git_path(): keep "info/sparse-checkout" per work-tree

 Documentation/config.txt               |   9 ++
 Documentation/git-checkout.txt         |  34 +++++
 Documentation/git-prune.txt            |   3 +
 Documentation/git-rev-parse.txt        |  10 ++
 Documentation/git.txt                  |   9 ++
 Documentation/gitrepository-layout.txt |  75 ++++++++--
 builtin/branch.c                       |   4 +-
 builtin/checkout.c                     | 248 ++++++++++++++++++++++++++++++++-
 builtin/clone.c                        |   9 +-
 builtin/commit.c                       |   2 +-
 builtin/count-objects.c                |   4 +-
 builtin/fetch.c                        |   5 +-
 builtin/fsck.c                         |   4 +-
 builtin/gc.c                           |  21 ++-
 builtin/init-db.c                      |   7 +-
 builtin/prune.c                        |  74 ++++++++++
 builtin/receive-pack.c                 |   2 +-
 builtin/reflog.c                       |   2 +-
 builtin/remote.c                       |   2 +-
 builtin/repack.c                       |   8 +-
 builtin/rev-parse.c                    |  11 ++
 cache.h                                |  17 ++-
 daemon.c                               |  11 +-
 environment.c                          |  45 ++++--
 fast-import.c                          |   7 +-
 git-am.sh                              |  22 +--
 git-pull.sh                            |   2 +-
 git-rebase--interactive.sh             |   6 +-
 git-rebase--merge.sh                   |   6 +-
 git-rebase.sh                          |   4 +-
 git-sh-setup.sh                        |   2 +-
 git-stash.sh                           |   6 +-
 git.c                                  |   2 +-
 notes-merge.c                          |   6 +-
 path.c                                 | 234 +++++++++++++++++++++----------
 refs.c                                 |  86 +++++++-----
 refs.h                                 |   2 +-
 run-command.c                          |   4 +-
 run-command.h                          |   2 +-
 setup.c                                | 124 +++++++++++++----
 sha1_file.c                            |   2 +-
 submodule.c                            |   9 +-
 t/t0060-path-utils.sh                  |  35 +++++
 t/t1501-worktree.sh                    |  76 ++++++++++
 t/t1510-repo-setup.sh                  |   1 +
 t/t2025-checkout-to.sh (new +x)        |  72 ++++++++++
 templates/hooks--applypatch-msg.sample |   4 +-
 templates/hooks--pre-applypatch.sample |   4 +-
 trace.c                                |   1 +
 transport.c                            |   8 +-
 wrapper.c                              |  31 +++++
 51 files changed, 1109 insertions(+), 265 deletions(-)
 create mode 100755 t/t2025-checkout-to.sh