Commits, trees and tags have structure. Don't let users feed git
with malformed ones. Sooner or later git will die() when
encountering them.
Note that this patch does not check semantics. A tree that points
to non-existent objects is perfectly OK (and should be so, users
may choose to add commit first, then its associated tree for example).
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>