git-gui: Narrow the no differences information message.

git-gui: Narrow the no differences information message.

On Mac OS X the no differences informational message was linewrapped
at the wrong points due to the limited width of the system dialog,
yet the LFs embedded in the message (where I linewrapped it manually)
were also being honored.  This resulted in a very difficult to read
paragraph of text.

So this narrows the text down by another 10 columns or so, making it
more readable.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>