gitk: Use the -q flag to git checkout

This avoids having gitk think that an error has occurred in the checkout.

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/gitk b/gitk
index a57e84c..3c9ea97 100755
--- a/gitk
+++ b/gitk
@@ -5356,7 +5356,7 @@
     nowbusy checkout
     update
     if {[catch {
-	exec git checkout $headmenuhead
+	exec git checkout -q $headmenuhead
     } err]} {
 	notbusy checkout
 	error_popup $err