remove use of "tail -n 1" and "tail -1"

The "-n" syntax is not supported by System V versions of
tail (which prefer "tail -1"). Unfortunately "tail -1" is
not actually POSIX.  We had some of both forms in our
scripts.

Since neither form works everywhere, this patch replaces
both with the equivalent sed invocation:

sed -ne '$p'

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>