Replace non-trivial uses of RefDatabase.getRefs with getRefsByPrefix

getRefs returns Map<String, Ref> where the key is the ref name with the
given prefix stripped off. However, getRefsByPrefix returns a list of
Refs with the full refname.

Therefore, when converting to getRefsByPrefix, we have to explicitly
remove the ref prefix from each ref name before using it.

Change-Id: I9a3ecf942ea47325fffa27596265fde217c2af0c

java/com/google/gerrit/server/StarredChangesUtil.java

@@ -296,7 +296,11 @@ public class StarredChangesUtil {
 
   private static Set<String> getRefNames(Repository repo, String prefix) throws IOException {
     RefDatabase refDb = repo.getRefDatabase();
-    return refDb.getRefs(prefix).keySet();
+    return refDb
+        .getRefsByPrefix(prefix)
+        .stream()
+        .map(r -> r.getName().substring(prefix.length()))
+        .collect(toSet());
   }
 
   public ObjectId getObjectId(Account.Id accountId, Change.Id changeId) {