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/advice.h b/advice.h
index c28ef8a..8bf6356 100644
--- a/advice.h
+++ b/advice.h
@@ -7,6 +7,7 @@
 extern int advice_push_non_ff_current;
 extern int advice_push_non_ff_default;
 extern int advice_push_non_ff_matching;
+extern int advice_push_already_exists;
 extern int advice_status_hints;
 extern int advice_commit_before_merge;
 extern int advice_resolve_conflict;