notmuch.el: Override next-line and previous-line to make them reliable.

I noticed that these functions would sometimes leave point on an
invisible character[*]. The problem would be that point would appear
to be on a particular message, but adding or removing a tag would
actually add/remove a tag from the *previous* message.

Fix the C-n and C-p keybindings at least to call the underlying
command and then advance to a visible character. We set this-command
in our overrides so that the temporary-goal-column feature still
works.

[*] The documentation says that command loop is supposed to move point
outside of any invisible region when a command exits. But apparently
not.