Fix org-habit bug related to string mutation

* lisp/org/org-habit.el (org-habit-build-graph): Rewrite without using
string mutation (using vectors instead), fixing a bug where
org-habit-completed-glyph and org-habit-today-glyph wouldn't display
properly if in the U+0080..00FF range, discovered by the more restricted
string mutation.

Reported by Daniel Mendler in
 https://lists.gnu.org/archive/html/emacs-orgmode/2025-08/msg00224.html

1 files changed, 14 insertions, 10 deletions

diff --git a/lisp/org/org-habit.el b/lisp/org/org-habit.el
index 38975682152..010c9daa00e 100644
--- a/lisp/org/org-habit.el
+++ b/lisp/org/org-habit.el
@@ -333,7 +333,8 @@ current time."
          (start (time-to-days starting))
          (now (time-to-days current))
          (end (time-to-days ending))
-         (graph (make-string (1+ (- end start)) ?\s))
+         (graph (make-vector (1+ (- end start)) ?\s))
+         (props nil)
          (index 0)
          last-done-date)
     (while (and done-dates (< (car done-dates) start))
@@ -411,17 +412,20 @@ current time."
                    (not (eq face 'org-habit-overdue-face))
                    (not markedp))
           (setq face (cdr faces)))
-        (put-text-property index (1+ index) 'face face graph)
-        (put-text-property index (1+ index)
-                           'help-echo
-                           (concat (format-time-string
-                                    (org-time-stamp-format)
-                                    (time-add starting (days-to-time (- start (time-to-days starting)))))
-                                   (if donep " DONE" ""))
-                           graph))
+        (push (list index (1+ index) 'face face) props)
+        (push (list index (1+ index)
+                    'help-echo
+                    (concat (format-time-string
+                             (org-time-stamp-format)
+                             (time-add starting (days-to-time (- start (time-to-days starting)))))
+                            (if donep " DONE" "")))
+              props))
       (setq start (1+ start)
             index (1+ index)))
-    graph))
+    (let ((graph-str (concat graph)))
+      (dolist (p props)
+        (put-text-property (nth 0 p) (nth 1 p) (nth 2 p) (nth 3 p) graph-str))
+      graph-str)))
 
 (defun org-habit-insert-consistency-graphs (&optional line)
   "Insert consistency graph for any habitual tasks."