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>

git-rebase--interactive.sh

@@ -218,7 +218,7 @@ nth_string () {
 make_squash_message () {
 	if test -f "$SQUASH_MSG"; then
 		COUNT=$(($(sed -n "s/^# This is [^0-9]*\([1-9][0-9]*\).*/\1/p" \
-			< "$SQUASH_MSG" | tail -n 1)+1))
+			< "$SQUASH_MSG" | sed -ne '$p')+1))
 		echo "# This is a combination of $COUNT commits."
 		sed -e 1d -e '2,/^./{
 			/^$/d