git-gui: Disable the text widget in the repository chooser

Although we are using a text widget here we really do not
want the end-user to be able to modify the text it displays.
So we need to disable it.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

lib/choose_repository.tcl

@@ -108,6 +108,8 @@ constructor pick {} {
 			-label [mc "Open..."]
 	}
 
+	$opts conf -state disabled
+
 	set sorted_recent [_get_recentrepos]
 	if {[llength $sorted_recent] > 0} {
 		if {$m_repo ne {}} {