bug#37515: 27.0.50; button.el doesn't support help-echo function

[Katsumi Yamaoka <yamaoka@jpl.org>]

Hi,

I found the `forward-button' command shows a Lisp function object
if the value of the `help-echo' text property at the destination
point is a function.  But is should be its return value.

,---- (info "(elisp) Special Properties")
| `help-echo'
|  If text has a string as its `help-echo' property, then when you
|  move the mouse onto that text, Emacs displays that string in the
|  echo area, or in the tooltip window (*note Tooltips::).
|
|  If the value of the `help-echo' property is a function, that
|  function is called with three arguments, WINDOW, OBJECT and POS and
|  should return a help string or `nil for none.
`----

So, `forward-button' should be fixed to something like this:

--8<---------------cut here---------------start------------->8---
--- button.el~	2019-07-30 21:20:12.288879200 +0000
+++ button.el	2019-09-26 01:12:28.492808400 +0000
@@ -513,8 +513,11 @@
             nil
 	  (user-error (if wrap "No buttons!" "No more buttons")))
       (let ((msg (and display-message (button-get button 'help-echo))))
-	(when msg
-	  (message "%s" msg)))
+	(cond ((functionp msg)
+	       (message
+		"%s" (funcall msg (selected-window) (current-buffer) (point))))
+	      (msg
+	       (message "%s" msg))))
       button)))
 
 (defun backward-button (n &optional wrap display-message no-error)
--8<---------------cut here---------------end--------------->8---

Thanks.