Fwd: Git clone fails during pre-commit hook due to GIT_WORK_TREE=. (regression 2.5 -> 2.6)

[Anthony Sottile <asottile@umich.edu>]

* Short description of the problem *

It seems GIT_WORK_DIR is now exported invariantly when calling git
hooks such as pre-commit.  If these hooks involve cloning repositories
they will not fail due to this exported environment variable.  This
was not the case in prior versions (such as v2.5.0).

* Simple reproduction *

```
$ cat test.sh
#!/usr/bin/env bash
set -ex

rm -rf test

# Exit non {0, 1} to abort git bisect
make -j 8 > /dev/null || exit 2

# Put our new git on the path
PATH="$(pwd):$PATH"

git init test

pushd test
mkdir -p .git/hooks
echo 'git clone git://github.com/asottile/css-explore css-explore' >
.git/hooks/pre-commit
chmod 755 .git/hooks/pre-commit

git commit -m foo --allow-empty || exit 1
```

* Under 2.6.3 *

```
$ ./test.sh

...

+ git init test
warning: templates not found /home/anthony/share/git-core/templates
Initialized empty Git repository in /home/anthony/workspace/git/test/.git/
+ pushd test
~/workspace/git/test ~/workspace/git
+ mkdir -p .git/hooks
+ echo 'git clone git://github.com/asottile/css-explore css-explore'
+ chmod 755 .git/hooks/pre-commit
+ git commit -m foo --allow-empty
fatal: working tree '.' already exists.
+ exit 1
```

* Under 2.5 *

```
$ ./test.sh

...

+ git init test
warning: templates not found /home/anthony/share/git-core/templates
Initialized empty Git repository in /home/anthony/workspace/git/test/.git/
+ pushd test
~/workspace/git/test ~/workspace/git
+ mkdir -p .git/hooks
+ echo 'git clone git://github.com/asottile/css-explore css-explore'
+ chmod 755 .git/hooks/pre-commit
+ git commit -m foo --allow-empty
Cloning into 'css-explore'...
warning: templates not found /home/anthony/share/git-core/templates
remote: Counting objects: 214, done.
remote: Total 214 (delta 0), reused 0 (delta 0), pack-reused 214
Receiving objects: 100% (214/214), 25.89 KiB | 0 bytes/s, done.
Resolving deltas: 100% (129/129), done.
Checking connectivity... done.
[master (root-commit) 5eb999d] foo
```


* Bisect *

```
$ git bisect good v2.5.0
$ git bisect bad origin/master
$ git bisect run ./test.sh

...

d95138e695d99d32dcad528a2a7974f434c51e79 is the first bad commit
commit d95138e695d99d32dcad528a2a7974f434c51e79
Author: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Date:   Fri Jun 26 17:37:35 2015 +0700

    setup: set env $GIT_WORK_TREE when work tree is set, like $GIT_DIR

    In the test case, we run setup_git_dir_gently() the first time to read
    $GIT_DIR/config so that we can resolve aliases. We'll enter
    setup_discovered_git_dir() and may or may not call set_git_dir() near
    the end of the function, depending on whether the detected git dir is
    ".git" or not. This set_git_dir() will set env var $GIT_DIR.

    For normal repo, git dir detected via setup_discovered_git_dir() will be
    ".git", and set_git_dir() is not called. If .git file is used however,
    the git dir can't be ".git" and set_git_dir() is called and $GIT_DIR
    set. This is the key of this problem.

    If we expand an alias (or autocorrect command names), then
    setup_git_dir_gently() is run the second time. If $GIT_DIR is not set in
    the first run, we run the same setup_discovered_git_dir() as before.
    Nothing to see. If it is, however, we'll enter setup_explicit_git_dir()
    this time.

    This is where the "fun" is.  If $GIT_WORK_TREE is not set but
    $GIT_DIR is, you are supposed to be at the root level of the
    worktree.  But if you are in a subdir "foo/bar" (real worktree's top
    is "foo"), this rule bites you: your detected worktree is now
    "foo/bar", even though the first run correctly detected worktree as
    "foo". You get "internal error: work tree has already been set" as a
    result.

    Bottom line is, when $GIT_DIR is set, $GIT_WORK_TREE should be set too
    unless there's no work tree. But setting $GIT_WORK_TREE inside
    set_git_dir() may backfire. We don't know at that point if work tree is
    already configured by the caller. So set it when work tree is
    detected. It does not harm if $GIT_WORK_TREE is set while $GIT_DIR is
    not.

    Reported-by: Bjørnar Snoksrud <snoksrud@gmail.com>
    Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>

:100644 100644 9daa0ba4a36ced9f63541203e7bcc2ab9e1eae56
36fbba57fc83afd36d99bf5d4f3a1fc3feefba09 M    environment.c
:040000 040000 1d7c4bf77e0fd49ca315271993cb69a8b055c3aa
145d85895cb6cb0810597e1854a7721ccfc8f457 M    t
bisect run success
```

Causing me a few headaches in
https://github.com/pre-commit/pre-commit/issues/300
I'm working around it in https://github.com/pre-commit/pre-commit/pull/301

Thanks,

Anthony