Document why ReplicationUser doesn't use registered groups

I forgot why we did this isEmpty test before adding the registered
groups into the set.  Explain what the heck is going on.

Change-Id: Ic62ec8b56b139aa3e9276fc9f3c9b2d3da12273c
Signed-off-by: Shawn O. Pearce <sop@google.com>

gerrit-server/src/main/java/com/google/gerrit/server/ReplicationUser.java

@@ -29,24 +29,34 @@ public class ReplicationUser extends CurrentUser {
     ReplicationUser create(@Assisted Set<AccountGroup.Id> authGroups);
   }
 
-  private Set<AccountGroup.Id> effectiveGroups;
+  private final Set<AccountGroup.Id> effectiveGroups;
 
   @Inject
   protected ReplicationUser(AuthConfig authConfig,
       @Assisted Set<AccountGroup.Id> authGroups) {
     super(AccessPath.REPLICATION, authConfig);
-    effectiveGroups = new HashSet<AccountGroup.Id>(authGroups);
 
-    if (effectiveGroups.isEmpty()) {
+    if (authGroups.isEmpty()) {
-      effectiveGroups.addAll(authConfig.getRegisteredGroups());
+      // Only include the registered groups if no specific groups
+      // were provided. This allows an administrator to configure
+      // a replication user with a narrower view of the system than
+      // all other users, such as when replicating from an internal
+      // company server to a public open source distribution site.
+      //
+      effectiveGroups = authConfig.getRegisteredGroups();
+
+    } else {
+      effectiveGroups = copy(authGroups);
+    }
   }
 
-    effectiveGroups = Collections.unmodifiableSet(effectiveGroups);
+  private static Set<AccountGroup.Id> copy(Set<AccountGroup.Id> groups) {
+    return Collections.unmodifiableSet(new HashSet<AccountGroup.Id>(groups));
   }
 
   @Override
   public Set<AccountGroup.Id> getEffectiveGroups() {
-    return Collections.unmodifiableSet(effectiveGroups);
+    return effectiveGroups;
   }
 
   @Override