stash apply: report status correctly even in a worktree's subdirectory

stash apply: report status correctly even in a worktree's subdirectory

When Git wants to spawn a child Git process inside a worktree's
subdirectory while `GIT_DIR` is set, we need to take care of specifying
the work tree's top-level directory explicitly because it cannot be
discovered: the current directory is _not_ the top-level directory of
the work tree, and neither is it inside the parent directory of
`GIT_DIR`.

This fixes the problem where `git stash apply` would report pretty much
everything deleted or untracked when run inside a worktree's
subdirectory.

To make sure that we do not introduce the "reverse problem", i.e. when
`GIT_WORK_TREE` is defined but `GIT_DIR` is not, we simply make sure
that both are set.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>