org-habit: Add org-habit-scheduled-past-days

* lisp/org-habit.el (org-habit-scheduled-past-days): New variable

* lisp/org-agenda.el (org-agenda-get-scheduled): override
  `org-scheduled-past-days' for habits if
  `org-habit-scheduled-past-days` is not nil

TINYCHANGE

lisp/org-agenda.el

@@ -90,6 +90,7 @@
 (defvar org-habit-show-habits)
 (defvar org-habit-show-habits-only-for-today)
 (defvar org-habit-show-all-today)
+(defvar org-habit-scheduled-past-days)
 
 ;; Defined somewhere in this file, but used before definition.
 (defvar org-agenda-buffer-name "*Org Agenda*")
@@ -6201,7 +6202,8 @@ scheduled items with an hour specification like [h]h:mm."
 		       habitp
 		       (bound-and-true-p org-habit-show-all-today))
 	    (when (or (and (> ddays 0) (< diff ddays))
-		      (> diff org-scheduled-past-days)
+		      (> diff (or (and habitp org-habit-scheduled-past-days)
+				  org-scheduled-past-days))
 		      (> schedule current)
 		      (and (/= current schedule)
 			   (/= current today)

lisp/org-habit.el

@@ -89,6 +89,21 @@ It will be green even if it was done after the deadline."
   :group 'org-habit
   :type 'boolean)
 
+(defcustom org-habit-scheduled-past-days nil
+"Value to use instead of `org-scheduled-past-days', for habits only.
+
+If nil, `org-scheduled-past-days' is used.
+
+Setting this to say 10000 is a way to make habits always show up
+as a reminder, even if you set `org-scheduled-past-days' to a
+small value because you regard scheduled items as a way of
+\"turning on\" TODO items on a particular date, rather than as a
+means of creating calendar-based reminders."
+  :group 'org-habit
+  :type '(choice integer (const nil))
+  :package-version '(Org . "9.3")
+  :safe (lambda (v) (or (integerp v) (null v))))
+
 (defface org-habit-clear-face
   '((((background light)) (:background "#8270f9"))
     (((background dark)) (:background "blue")))