setup: rework setup_explicit_git_dir()

setup: rework setup_explicit_git_dir()

This function is the most complex one among the three setup_*
functions because all GIT_DIR, GIT_WORK_TREE, core.worktree and
core.bare are involved.

Because core.worktree is only effective inside
setup_explicit_git_dir() and the extra code in setup_git_directory()
is to handle that. The extra code can now be retired.

Also note that setup_explicit assignment is removed, worktree setting
is no longer decided by get_git_work_tree(). get_git_work_tree() will
be simplified in the next commit.

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