bug#37729: "!${PATH}/date" path prefix in DEMO change directory is failing

[Kevin J. McCarthy]

I'm completely unfamiliar with the code, and just an elisp novice.  But 
I've been playing around.

Making this change seems to have fixed the problem for me.  I have no 
idea if this is correct outside of my little test in DEMO.

modified   hbut.el:
@@ -1224,6 +1224,7 @@ excluding delimiters, not just one."
                  (hattr:get 'hbut:current 'lbl-key)
                (hattr:set 'hbut:current 'loc (save-excursion
                                                (hbut:key-src 'full)))
+               (hattr:set 'hbut:current 'dir default-directory)
                (hattr:set 'hbut:current 'categ is-type)
                (or (hattr:get 'hbut:current 'args)
                    (not (listp args))