git-gui: Avoid console scrollbars unless they are necessary

git-gui: Avoid console scrollbars unless they are necessary

We shouldn't create scrollbars for the horziontal or vertical sides
unless there is enough content to make it worth drawing these widgets
on screen.  This way users don't loose screen space to objects that
won't help them navigate the display.

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