UploadPack: Avoid calling AdvertiseRefsHook twice

The AdvertiseRefsHook can be called twice if the following conditions
hold:

 1. This AdvertiseRefsHook doesn't set this.refs.
 2. getAdvertisedOrDefaultRefs is called after getFilteredRefs.

For example, this can happen when fetchV2 is called after lsRefsV2
when using a stateful bidirectional transport.

The second call does not accomplish anything useful.  Guard it with
'if (!advertiseRefsHookCalled)' to avoid wasted work.

Reported-by: Jonathan Tan <jonathantanmy@google.com>
Change-Id: Ib746582e4ef645b767a5b3fb969596df99ac2ab5
Signed-off-by: Jonathan Nieder <jrn@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java

@@ -790,8 +790,10 @@ public class UploadPack {
 			return refs;
 		}
 
+		if (!advertiseRefsHookCalled) {
 			advertiseRefsHook.advertiseRefs(this);
 			advertiseRefsHookCalled = true;
+		}
 		if (refs == null) {
 			// Fall back to all refs.
 			setAdvertisedRefs(