org-habit: C-u K in agenda toggles display all/today habits

* lisp/org-habit.el (org-habit-toggle-display-in-agenda): New command.

doc/org-manual.org

@@ -4187,7 +4187,8 @@ the way habits are displayed in the agenda.
 
   #+vindex: org-habit-show-habits-only-for-today
   If non-~nil~, only show habits in today's agenda view.  The default
-  value is ~t~.
+  value is ~t~.  Pressing {{{kbd(C-u K)}}} in the agenda toggles this
+  variable.
 
 Lastly, pressing {{{kbd(K)}}} in the agenda buffer causes habits to
 temporarily be disabled and do not appear at all.  Press {{{kbd(K)}}}

etc/ORG-NEWS

@@ -152,7 +152,10 @@ LaTeX font locking.
 negative value as a valid repeater to shift time stamps in backward
 in cloned subtrees.  You can give, for example, ‘-3d’ to shift three
 days in the past.
-
+*** Toggle display of all vs. undone scheduled habits conveniently
+=<C-u K>= (~org-habit-toggle-display-in-agenda~) in an agenda toggles
+the display of all habits to those which are undone and scheduled.
+This is a function for convenience.
 ** New functions
 *** ~org-dynamic-block-insert-dblock~
 
@@ -164,6 +167,7 @@ dynamic block in ~org-dynamic-block-alist~.
 *** ~org-table-cell-down~
 *** ~org-table-cell-left~
 *** ~org-table-cell-right~
+*** ~org-habit-toggle-display-in-agenda~
 ** Removed functions
 *** ~org-babel-set-current-result-hash~
 

lisp/org-habit.el

@@ -449,7 +449,18 @@ current time."
   (message "Habits turned %s"
 	   (if org-habit-show-habits "on" "off")))
 
-(org-defkey org-agenda-mode-map "K" 'org-habit-toggle-habits)
+(defun org-habit-toggle-display-in-agenda (arg)
+  "Toggle display of habits in agenda.
+With ARG toggle display of all vs. undone scheduled habits.
+See `org-habit-show-all-today'."
+  (interactive "P")
+  (if (not arg)
+      (org-habit-toggle-habits)
+    (org-agenda-check-type t 'agenda)
+    (setq org-habit-show-all-today (not org-habit-show-all-today))
+    (when org-habit-show-habits (org-agenda-redo))))
+
+(org-defkey org-agenda-mode-map "K" 'org-habit-toggle-display-in-agenda)
 
 (provide 'org-habit)