Freeze break: gnome-shell: First chat message shows up twice

[Owen Taylor <otaylor redhat com>]

https://bugzilla.gnome.org/show_bug.cgi?id=645612

prior to 2.91.92, GNOME Shell's chat bubbles were enhanced to correctly
retrieve old history from Telepathy. A bug in that patch resulted in
duplication of the first new line - it would appear both twice - once
from the log and once from being received as a new message.

Patch on the bug is relatively simple, local. Reviewed by
Jasper St. Pierre who wrote the code originally, double checked by me.

- Owen