notmuch.el: Un-break the magic space bar to scroll a long, single message.

Clearly some recent code was very fragile, which I noticed in that the
space bar would no longer scroll a long message if it was the only
message in a thread.

This resulted in a lot of churn, but hopefully things are more robust
now, (for example by using new predicates like
notmuch-show-last-message-p rather than doing heuristics based on
(eobp) or (window-end)).

As usual, the presence of invisible characters complicates the task of
making this stuff robust.