UploadPack: add getProtocolV2Hook() method

A caller cannot install a second hook in the UploadPack without
overwriting whatever is already there.

Offer a method to get the current protocol v2 hook, so it can be chained
with new hooks.

Change-Id: Icb06f94ec52b8c8714f509b5b8622d6db42960fb
Signed-off-by: Ivan Frade <ifrade@google.com>

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

@@ -601,6 +601,18 @@ public class UploadPack {
 		this.protocolV2Hook = hook != null ? hook : ProtocolV2Hook.DEFAULT;
 	}
 
+	/**
+	 * Get the currently installed protocol v2 hook.
+	 *
+	 * @return the hook or a default implementation if none installed.
+	 *
+	 * @since 5.5
+	 */
+	public ProtocolV2Hook getProtocolV2Hook() {
+		return this.protocolV2Hook != null ? this.protocolV2Hook
+				: ProtocolV2Hook.DEFAULT;
+	}
+
 	/**
 	 * Set the filter used while advertising the refs to the client.
 	 * <p>