git-gui: store wm state and fix wm geometry

git-gui: store wm state and fix wm geometry

I often close git gui window when it is maximized, and when I reopen
it next time the it would usually become out of place (e.g. a huge
window with a top-left corner somewhere close to the center of the
screen). Fix it by storing and restoring wm state in config, as well
as setting wm state to normal before retrieving wm geometry info.

Signed-off-by: Alexey Borzenkov <snaury@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>