Description:
When repeat-mode is enabled and a repeat-exit-timeout is set, the
echo-area is not cleared in case the echo-area has some content and a
single repeat-command is executed.

Steps:
1) emacs -Q (from master branch)
2) eval
    (setq repeat-exit-timeout 1)
    (repeat-mode 1)
3) create some windows with 'C-x 2' and 'C-x 3'
4) 'C-x'
5) wait prefix "C-x" to appear on echo-area
6) 'o' (echo-area will now contain "C-x o [Repeat with o, O]")
7) wait a few seconds so 'repeat-exit-timeout' takes effect
Expected: the echo-area is cleared
Actual: the echo-area is not cleared

If 'o' is pressed again, it will run 'self-insert-command' even though
the echo-area is still showing "C-x o [Repeat with o, O]".

Now repeat steps 1-5 but press 'o' quickly to invoke the repeat command
multiple times (which will trigger other-window multiple times). The
echo-area changes to "Repeat o, O" and is properly cleaned after
'repeat-exit-timeout' seconds.

Patch:
I created a simple patch that seems to fix this probem. The
'string-prefix-p' was replaced with 'string-match-p' to handle cases
where echo-area contains some other message. This check seems very poor,
though, so any better approach is welcome.