setup.c: re-fix d95138e (setup: set env $GIT_WORK_TREE when ..

setup.c: re-fix d95138e (setup: set env $GIT_WORK_TREE when ..

Commit d95138e [1] attempted to fix a .git file problem by
setting GIT_WORK_TREE whenever GIT_DIR is set. It sounded harmless
because we handle GIT_DIR and GIT_WORK_TREE side by side for most
commands, with two exceptions: git-init and git-clone.

"git clone" is not happy with d95138e. This command ignores GIT_DIR
but respects GIT_WORK_TREE [2] [3] which means it used to run fine
from a hook, where GIT_DIR was set but GIT_WORK_TREE was not (*).
With d95138e, GIT_WORK_TREE is set all the time and git-clone
interprets that as "I give you order to put the worktree here",
usually against the user's intention.

The solution in d95138e is reverted earlier, and instead we reuse
the solution from c056261 [4].  It fixed another setup-messed-
up-by-alias by saving and restoring env and spawning a new process,
but for git-clone and git-init only.

Now we conclude that setup-messed-up-by-alias is always evil. So the
env restoration is done for _all_ commands, including external ones,
whenever aliases are involved. It fixes what d95138e tried to fix,
without upsetting git-clone-inside-hooks.

The test from d95138e remains to verify it's not broken by this. A new
test is added to make sure git-clone-inside-hooks remains happy.

(*) GIT_WORK_TREE was not set _most of the time_. In some cases
    GIT_WORK_TREE is set and git-clone will behave differently. The
    use of GIT_WORK_TREE to direct git-clone to put work tree
    elsewhere looks like a mistake because it causes surprises this
    way. But that's a separate story.

[1] d95138e (setup: set env $GIT_WORK_TREE when work tree is set, like
             $GIT_DIR - 2015-06-26)
[2] 2beebd2 (clone: create intermediate directories of destination
             repo - 2008-06-25)
[3] 20ccef4 (make git-clone GIT_WORK_TREE aware - 2007-07-06)
[4] c056261 (git potty: restore environments after alias expansion -
             2014-06-08)

Reported-by: Anthony Sottile <asottile@umich.edu>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>