Add missing inside_work_tree setting in setup_git_directory_gently

Add missing inside_work_tree setting in setup_git_directory_gently

When both GIT_DIR and GIT_WORK_TREE are set, and
setup_git_directory_gently() changes the current working
directory accordingly, it should also set inside_work_tree = 1.

Without this, work_tree handling code in setup_git_directory()
will be activated. If you stay in root work tree (no prefix),
it does not harm. It does if you work from a subdirectory though.

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