emacs: tag-jump: make k binding for the reverse tag change map

Currently, by default k invokes the tag-jump menu, and following it by
r invokes the reverse tag change jump menu. This is awkward to type
(e.g. k r u for undoing a -unread change). This changes it so that k
followed by k invokes the reverse menu. We make the key for the
reverse map a variable as that makes it possible for a user to
change it by editing their .emacs file.