git-gui: Default selection to first matching ref

git-gui: Default selection to first matching ref

If we have specifications listed in our revision picker mega-widget
then we should default the selection within that widget to the first
ref available.  This way the user does not need to use the spacebar
to activate the selection of a ref within the box; instead they can
navigate up/down with the arrow keys and be done with it.

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