emacs: Unbreak notmuch-show-rewind (the function to which Backspace is bound)

Commit 095a02211e696434e5b41a85ab516b3a639f9a9b broke the backspace
key by trying to execute nil as a function, (which obviously won't
work), when it was intended as a return value. Fix this now, (and pine
for a test suite that exercises the emacs user-interface of notmuch).