t5004: fix issue with empty archive test and bsdtar

t5004: fix issue with empty archive test and bsdtar

bsdtar, which is the default tar on Mac OS X, handles empty archives
just fine but reports archives containing only a pax extended header
comment as damaged.  Work around the issue by explicitly generating
the archive for the tree and not the commit, which causes git archive
to omit the commit hash comment record from the tar file.

Reported-by: BJ Hargrave <bj@bjhargrave.com>
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>