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>

lib/branch_checkout.tcl

@@ -30,6 +30,7 @@ constructor dialog {} {
 	pack $w.buttons -side bottom -fill x -pady 10 -padx 10
 
 	set w_rev [::choose_rev::new $w.rev {Revision}]
+	$w_rev bind_listbox <Double-Button-1> [cb _checkout]
 	pack $w.rev -anchor nw -fill both -expand 1 -pady 5 -padx 5
 
 	labelframe $w.options -text {Options}