Set default "tar" umask to 002 and owner.group to root.root

Set default "tar" umask to 002 and owner.group to root.root

In order to make the generated tar files more friendly to users who
extract them as root using GNU tar and its implied -p option, change
the default umask to 002 and change the owner name and group name to
root.  This ensures that a) the extracted files and directories are
not world-writable and b) that they belong to user and group root.

Before they would have been assigned to a user and/or group named
git if it existed.  This also answers the question in the removed
comment: uid=0, gid=0, uname=root, gname=root is exactly what we
want.

Normal users who let tar apply their umask while extracting are
only affected if their umask allowed the world to change their
files (e.g. a umask of zero).  This case is so unlikely and strange
that we don't need to support it.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>