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.

emacs/notmuch-tag.el

@@ -485,6 +485,9 @@ begin with a \"+\" or a \"-\". If REVERSE is non-nil, replace all
 		s)))
 	  tags))
 
+(defvar notmuch-tag-jump-reverse-key "k"
+  "The key in tag-jump to switch to the reverse tag changes.")
+
 (defun notmuch-tag-jump (reverse)
   "Create a jump menu for tagging operations.
 
@@ -523,7 +526,8 @@ and vice versa."
 	(push (list key name-string
 		     `(lambda () (,tag-function ',tag-change)))
 	      action-map)))
-    (push (list "r" (if reverse
+    (push (list notmuch-tag-jump-reverse-key
+		(if reverse
 		    "Forward tag changes "
 		  "Reverse tag changes")
 		(apply-partially 'notmuch-tag-jump (not reverse)))