git-gui: Allow double-click in checkout dialog to start checkout

git-gui: Allow double-click in checkout dialog to start checkout

If the user double clicks a branch in the checkout dialog then they
probably want to start the checkout process on that branch.  I found
myself doing this without realizing it, and of course it did nothing
as there was no action bound to the listbox's Double-Button-1 event
handler.  Since I did it without thinking, others will probably also
try, and expect the same behavior.

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