emacs/show: force notmuch-show-buttonise-links to act on lines

This seems to fix a problem with emacs 25 creating partial buttons by
calling n-s-b-l with a region that does not include the whole button.
I'm not 100% sure it's legit to act outside the region passed by
jit-lock, but goto-address-fontify-region (where I borrowed the code
from) already does this, so this patch to not make things worse.