Avoid one-or-more (\+) non BRE in sed scripts.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -190,7 +190,7 @@ nth_string () {
 
 make_squash_message () {
 	if test -f "$SQUASH_MSG"; then
-		COUNT=$(($(sed -n "s/^# This is [^0-9]*\([0-9]\+\).*/\1/p" \
+		COUNT=$(($(sed -n "s/^# This is [^0-9]*\([1-9][0-9]*\).*/\1/p" \
 			< "$SQUASH_MSG" | tail -n 1)+1))
 		echo "# This is a combination of $COUNT commits."
 		sed -n "2,\$p" < "$SQUASH_MSG"