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>

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