path.c: make get_pathname() return strbuf instead of static buffer

We've been avoiding PATH_MAX whenever possible. This patch makes
get_pathname() return a strbuf and updates the callers to take
advantage of this. The code is simplified as we no longer need to
worry about buffer overflow.
Convert git_snpath() to strbuf_git_path()

In the previous patch, git_snpath() is modified to take a strbuf
buffer from get_pathname() because vsnpath() needs that. But that
makes it awkward because git_snpath() receives a pre-allocated buffer
from outside and has to copy data back.

Rename it to strbuf_git_path() and make it receive strbuf directly.
path.c: rename vsnpath() to do_git_path()

The name vsnpath() gives an impression that this is general path
handling function. It's not. This is the underlying implementation of
git_path(), git_pathdup() and strbuf_git_path() which will prefix
$GIT_DIR in the result string.
path.c: group git_path(), git_pathdup() and strbuf_git_path() together
Make git_path() aware of file relocation in $GIT_DIR

We allow the user to relocate certain paths out of $GIT_DIR via
environment variables, e.g. GIT_OBJECT_DIRECTORY, GIT_INDEX_FILE and
GIT_GRAFT_FILE. All callers are not supposed to use git_path() or
git_pathdup() to get those paths. Instead they must use
get_object_directory(), get_index_file() and get_graft_file()
respectively. This is inconvenient and could be missed in review
(there's git_path("objects/info/alternates") somewhere in
sha1_file.c).

This patch makes git_path() and git_pathdup() understand those
environment variables. So if you set GIT_OBJECT_DIRECTORY to /foo/bar,
git_path("objects/abc") should return /tmp/bar/abc. The same is done
for the two remaining env variables.

"git rev-parse --git-path" is the wrapper of this function for scripts
to use. For third party scripts that want to support old versions as
well, something like this as a wrapper might help:

  git_path() { git rev-parse --git-path "$1" || echo "$GIT_DIR/$1"; }
*.sh: respect $GIT_INDEX_FILE
reflog: avoid constructing .lock path with git_path

git_path() soon understands the path given to it. Some paths "abc" may
become "def" while other "ghi" may become "ijk". We don't want
git_path() to interfere with .lock path construction. Concatenate
".lock" after the path has been resolved by git_path() so if "abc"
becomes "def", we'll have "def.lock", not "ijk".
fast-import: use git_path() for accessing .git dir instead of get_git_dir()

This allows git_path() to redirect info/fast-import to another place
if needed
commit: use SEQ_DIR instead of hardcoding "sequencer"
Add new environment variable $GIT_COMMON_DIR

This variable is intended to support multiple working directories
attached to a repository. Such a repository may have a main working
directory, created by either "git init" or "git clone" and one or more
linked working directories. These working directories and the main
repository share the same repository directory.

In linked working directories, $GIT_COMMON_DIR must be defined to point
to the real repository directory and $GIT_DIR points to an unused
subdirectory inside $GIT_COMMON_DIR. File locations inside the
repository are reorganized from the linked worktree view point:

 - worktree-specific such as HEAD, logs/HEAD, index, other top-level
   refs and unrecognized files are from $GIT_DIR.

 - the rest like objects, refs, info, hooks, packed-refs, shallow...
   are from $GIT_COMMON_DIR

Scripts are supposed to retrieve paths in $GIT_DIR with "git rev-parse
--git-path", which will take care of "$GIT_DIR vs $GIT_COMMON_DIR"
business.

Note that logs/refs/.tmp-renamed-log is used to prepare new reflog
entry and it's supposed to be on the same filesystem as the target
reflog file. This is not guaranteed true for logs/HEAD when it's
mapped to repos/xx/logs/HEAD because the user can put "repos"
directory on different filesystem. Don't mess with .git unless you
know what you're doing.

The redirection is done by git_path(), git_pathdup() and
strbuf_git_path(). The selected list of paths goes to $GIT_COMMON_DIR,
not the other way around in case a developer adds a new
worktree-specific file and it's accidentally promoted to be shared
across repositories (this includes unknown files added by third party
commands)

The list of known files that belong to $GIT_DIR are:

ADD_EDIT.patch BISECT_ANCESTORS_OK BISECT_EXPECTED_REV BISECT_LOG
BISECT_NAMES CHERRY_PICK_HEAD COMMIT_MSG FETCH_HEAD HEAD MERGE_HEAD
MERGE_MODE MERGE_RR NOTES_EDITMSG NOTES_MERGE_WORKTREE ORIG_HEAD
REVERT_HEAD SQUASH_MSG TAG_EDITMSG fast_import_crash_* logs/HEAD
next-index-* rebase-apply rebase-merge rsync-refs-* sequencer/*
shallow_*

Path mapping is NOT done for git_path_submodule(). Multi-checkouts are
not supported as submodules.
git-sh-setup.sh: use rev-parse --git-path to get $GIT_DIR/objects

If $GIT_COMMON_DIR is set, $GIT_OBJECT_DIRECTORY should be
$GIT_COMMON_DIR/objects, not $GIT_DIR/objects. Just let rev-parse
--git-path handle it.
*.sh: avoid hardcoding $GIT_DIR/hooks/...

If $GIT_COMMON_DIR is set, it should be $GIT_COMMON_DIR/hooks/, not
$GIT_DIR/hooks/. Just let rev-parse --git-path handle it.
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()

If the file "$GIT_DIR/commondir" exists, it contains the value of
$GIT_COMMON_DIR.
setup.c: convert check_repository_format_gently to use strbuf
setup.c: detect $GIT_COMMON_DIR check_repository_format_gently()
setup: support multi-checkout repo setup

The repo setup procedure is updated to detect $GIT_DIR/commondir and
set $GIT_COMMON_DIR properly.

The core.worktree is ignored when $GIT_DIR/commondir presents. This is
because "commondir" repos are intended for separate/linked checkouts
and pointing them back to a fixed core.worktree just does not make
sense.
wrapper.c: wrapper to open a file, fprintf then close
use new wrapper write_file() for simple file writing

This fixes common problems in these code about error handling,
forgetting to close the file handle after fprintf() fails, or not
printing out the error string..