Reassign symbolic ref list after calling put.

This is required since RefList.put returns a new RefList.

Change-Id: I717d75d6f6154a6e0dc7cde3b72b0a59c68d955c
Signed-off-by: Kevin Sawicki <kevin@github.com>

org.eclipse.jgit.storage.dht/src/org/eclipse/jgit/storage/dht/DhtRefDatabase.java

@@ -337,7 +337,7 @@ public class DhtRefDatabase extends RefDatabase {
 			RefList<DhtRef> sym = oldCache.sym;
 
 			if (ref.isSymbolic()) {
-				sym.put(ref);
+				sym = sym.put(ref);
 			} else {
 				int p = sym.find(refName);
 				if (0 <= p)