git-gui: adjust the minimum height of diff pane for shorter screen height

git-gui: adjust the minimum height of diff pane for shorter screen height

When the main window is maximized, if the screen height is shorter (e.g.
Netbook screen 1024x600), both the partial commit pane and the status bar
are hidden. The diff pane is resizable, so that it can use less vertical
height, allowing the overall window to be shorter and still display both
the entire commit pane and status bar.

Signed-off-by: Vietor Liu <vietor@vxwo.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>