pull: handle --verify-signatures for unborn branch

pull: handle --verify-signatures for unborn branch

We usually just forward the --verify-signatures option along to
git-merge, and trust it to do the right thing. However, when we are on
an unborn branch (i.e., there is no HEAD yet), we handle this case
ourselves without even calling git-merge. And in this code path, we do
not respect the verification option at all.

It may be more maintainable in the long run to call git-merge for the
unborn case. That would fix this bug, as well as prevent similar ones in
the future. But unfortunately it's not easy to do. As t5520.3
demonstrates, there are some special cases that git-merge does not
handle, like "git pull .. master:master" (by the time git-merge is
invoked, we've overwritten the unborn HEAD).

So for now let's just teach git-pull to handle this feature.

Reported-by: Felix Eckhofer <felix@eckhofer.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>