bug#56820: outline-minor-mode replacing the first character with an arrow

[Juri Linkov <juri@linkov.net>]

> Thanks  that fixed the default behaviour but
> outline-minor-mode-use-buttons option still has a weird bug because
> when you use it with modes like python-mode and ruby-mode it replaces
> the first character of the outline-regep which for the aforementioned
> modes is strings like "module", "class", "def" etc... so when it
> replaces the first character it renders the buffer unreadable. So it
> might be better to insert the arrows in front of the first character
> instead of replacing the first character.

This patch could help to alleviate the problem by keeping
the first character displayed on the outline button:

```
diff --git a/lisp/outline.el b/lisp/outline.el
index 857ac9562f..498ea6fad4 100644
--- a/lisp/outline.el
+++ b/lisp/outline.el
@@ -1006,7 +1018,8 @@ outline--make-button-overlay
         (put-text-property (point) (1+ (point)) 'face (plist-get icon 'face)))
       (when-let ((image (plist-get icon 'image)))
         (overlay-put o 'display image))
-      (overlay-put o 'display (plist-get icon 'string))
+      (overlay-put o 'display (concat (plist-get icon 'string)
+                                      (string (char-after (point)))))
       (overlay-put o 'face (plist-get icon 'face)))
     o))
 
```