emacs: Fix the References header in reply

In the new reply code, the References header gets inserted by
message.el using a function called message-shorten-references. Unlike
all the other header-inserting functions, it doesn't put a newline
after the header, causing the next header to end up on the same
line. In our case, this header happened to be User-Agent, so it's hard
to notice. This is probably a bug in message.el, but we need to work
around it.

This fixes the problem by wrapping message-shorten-references in a
function that inserts a newline after if necessary. This should
protect against the message.el bug being fixed in the future.