push: allow already-exists advice to be disabled

Add 'advice.pushAlreadyExists' option to disable the advice shown when
an update is rejected for a reference that is not allowed to update at
all (verses those that are allowed to fast-forward.)

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/push.c b/builtin/push.c
index cf5ecfa..8491e43 100644
--- a/builtin/push.c
+++ b/builtin/push.c
@@ -247,6 +247,8 @@
 
 static void advise_ref_already_exists(void)
 {
+	if (!advice_push_already_exists || !advice_push_update_rejected)
+		return;
 	advise(_(message_advice_ref_already_exists));
 }