Be more careful with objects directory permissions on clone

Be more careful with objects directory permissions on clone

Honour the setgid and umask when re-creating the objects directory
at the destination.

cpio in copy-pass mode aims to copy file permissions which causes this
problem and cannot be disabled. Be explicit by copying the directory
structure first, honouring the permissions at the destination, then copy
the files with 0444 permissions. This also avoids bugs in some versions
of cpio.

Signed-off-by: Mark Hills <mark@pogo.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>