bisect: fix another instance of eval'ed string

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>