Bug 541404 - improve UndoableCompoundTextChange with single change

When the compound change has only 1 change, it can be processed as a
basic change, using the "text change" strategy rather than the "text
set".
This goes into a simpler redraw loop, which prevent some issues with
code minings.

Change-Id: Ie54960195b89317e21b182e16704babd52d8d43b
Signed-off-by: Mickael Istria <mistria@redhat.com>