setup: limit get_git_work_tree()'s to explicit setup case only

setup: limit get_git_work_tree()'s to explicit setup case only

get_git_work_tree() takes input as core.worktree, core.bare,
GIT_WORK_TREE and decides correct worktree setting.

Unfortunately it does not do its job well. core.worktree and
GIT_WORK_TREE should only be taken into account, if GIT_DIR is set
(which is handled by setup_explicit_git_dir). For other setup cases,
only core.bare matters.

Add a temporary variable setup_explicit to adjust get_git_work_tree()
behavior as such. This variable will be gone once setup_* rework is
done.

Also remove is_bare_repository_cfg check in set_git_work_tree() to
ease the rework. We are going to check for core.bare and core.worktree
early before setting worktree. For example, if core.bare is true, no
need to set worktree.

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