rev-parse: --show-cdup

When --show-prefix is useful, sometimes it is easier to cd up to
the toplevel of the tree.  This is equivalent to:

git rev-parse --show-prefix | sed -e 's|[^/][^/]*|..|g'

but we do not have to invoke sed for that.

Signed-off-by: Junio C Hamano <junkio@cox.net>