"git restore --staged" without --source does not make much sense since
by default we restore from the index. Instead of copying the index to
itself, set the default source to HEAD in this case, yielding behavior
that matches "git reset -- <paths>".
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>