bisect--helper: string output variables together with "&&"

bisect--helper: string output variables together with "&&"

When doing:

eval "git bisect--helper --next-vars" | {
        while read line
        do
                echo "$line &&"
        done
        echo ':'
}

the result code comes from the last "echo ':'", not from running
"git bisect--helper --next-vars".

This patch gets rid of the need to string together the line from
the output of "git bisect--helper" with "&&" in the calling script
by making "git bisect--helper --next-vars" return output variables
already in that format.

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