bisect: fix another instance of eval'ed string

When there is nothing to be skipped, the output from
rev-list --bisect-vars was eval'ed without first being
strung together with &&; this is probably not a problem
as it is much less likely to be a bad input than the list
handcrafted by the filter_skip function, but it still is
a good discipline.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-bisect.sh

@@ -263,7 +263,13 @@ filter_skipped() {
 	_skip="$2"
 
 	if [ -z "$_skip" ]; then
-		eval_rev_list "$_eval"
+		eval_rev_list "$_eval" | {
+			while read line
+			do
+				echo "$line &&"
+			done
+			echo ':'
+		}
 		return
 	fi