Built-in git-get-tar-commit-id

By being an internal command git-get-commit-id can make use of
struct ustar_header and other stuff and stops wasting precious
disk space.

Note: I recycled one of the two "tar-tree" entries instead of
splitting that cleanup into a separate patch.

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