git-gui: Clear diff from viewer if the side changed.

git-gui: Clear diff from viewer if the side changed.

If the user switches the currently shown file from one side of the UI
to the other then how its diff is presented would be different.  And
leaving the old diff up is downright confusing.

Since the diff is probably not interesting to the user after the switch
we should just clear the diff viewer.  This saves the user time, as they
won't need to wait for us to reload the diff.

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