(repeat-echo-message-string): Reuse 'r-m-c' prompt formatting

(repeat-echo-message-string): Reuse 'r-m-c' prompt formatting

Now that 'repeat-mode' supports adding labels ("hints") next to
available keys, its prompt is very similar in its structure to
what we have in 'read-multiple-choice' (a list of labeled keys).
To make the two interfaces more consistent, reuse the prompt
formatting that 'read-multiple-choice' employs.

See short discussion at
https://lists.gnu.org/archive/html/emacs-devel/2024-05/msg00467.html

* lisp/repeat.el (repeat-echo-message-string): Use
'rmc--add-key-description' to format keys and their labels.