Compose key (<Multi_key>) sent directly to Emacs 23 rather than composing, but not with XMODIFIERS=@im=none

[Brady Garvin]

Steps to reproduce:

1. Run Emacs 23 from the dash or via the command line:
    $ emacs
or even
    $ emacs -Q
2. Switch to the scratch buffer, if necessary.
3. Press `<Multi_key>', then `c', then `,'.

Expected results:

A c with a cedilla (ç) appears.

Actual results:

Emacs complains in the mini-buffer that ``<Multi_key> is undefined''.
The `c' and `,' self-insert.

This problem began when I upgraded to Saucy.

Running emacs with the input method set to none in XMODIFIERS is a workaround:
    $ XMODIFIERS=@im=none emacs

Unlike other similar bug reports that Google turned up, xterm does not exhibit the same problem; only Emacs seems to be affected. Hence, I am reporting this under emacs23 to start with, though I would not be surprised if it ended up elsewhere.

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: emacs23 23.4+1-4ubuntu2
ProcVersionSignature: Ubuntu 3.11.0-12.19-generic 3.11.3
Uname: Linux 3.11.0-12-generic x86_64
ApportVersion: 2.12.5-0ubuntu2
Architecture: amd64
Date: Thu Oct 17 19:59:09 2013
InstallationDate: Installed on 2012-10-18 (364 days ago)
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Release amd64 (20121017.5)
MarkForUpload: True
SourcePackage: emacs23
UpgradeStatus: Upgraded to saucy on 2013-10-17 (0 days ago)