bug#61535: 29.0.60; choose-completion erases in-region buffer

[Juri Linkov <juri@linkov.net>]

0. emacs-29 -Q
1. type: (with-c C-M-i
2. type: M-down ... M-RET
3. check that the whole buffer was deleted before a selected
   completion candidate was inserted to the buffer:
   C-h v buffer-undo-list RET

Here is a possible fix for emacs-29:

diff --git a/lisp/simple.el b/lisp/simple.el
index c58acfe3adc..1924567cc3f 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -9882,7 +9882,8 @@ choose-completion
       (with-current-buffer buffer
         (choose-completion-string
          choice buffer
-         (or (and completion-use-base-affixes base-affixes)
+         (or (and (not completion-in-region-mode)
+                  completion-use-base-affixes base-affixes)
              base-position
              ;; If all else fails, just guess.
              (list (choose-completion-guess-base-position choice)))