bug#19875: 25.0.50; Setting an input-method-function breaks prefix show on the echo area

[Óscar Fuentes]

emacs -Q
M-x set-input-method ENTER spanish-prefix ENTER

Now let's suppose that we want to execute a command with a numeric
prefix (say 42). Let's press C-u and wait until it is displayed on the
echo area, then press `4' and wait, then press `2' and wait until it
is displayed too. The echo area shows:

C-u C-u 4 C-u 4 2

The problem is not limited to `set-input-method'. I discovered it with
key-chord-mode. I also have echo-keystrokes set to a low value, which
makes the problem more jarring.



In GNU Emacs 25.0.50.5 (x86_64-unknown-linux-gnu, X toolkit)
 of 2015-01-03 on qcore
Repository revision: 11ee7d8bf16b8ee4fcb3659e4b09686ed8b10590
Windowing system distributor `The X.Org Foundation', version 11.0.11600000
System Description:     Ubuntu 14.10

Configured using:
 `configure --without-toolkit-scroll-bars --with-x-toolkit=lucid'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG SOUND GSETTINGS NOTIFY LIBXML2 FREETYPE XFT
ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix