git-tar-tree: Simplify write_trailer()

We can write the trailer in one or at most two steps; it will always
fit within two blocks.  With the last caller of get_record() gone we
can get rid of it.

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