notmuch show: Don't hide a digital signature.

notmuch show: Don't hide a digital signature.

This was a bug that was introduced in copying the indexing code over
into notmuch-show.c. When indexing, we want to ignore the signature,
(it has no interesting terms). But when presenting the message, it's
important to present the signature to the user.

(And would be even better if we presented whether or not the signature
is good.)