--separate-git-dir tells git to create git dir at the specified
location, instead of where it is supposed to be. A .git file that
points to that location will be put in place so that it appears normal
to repo discovery process.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>