RefUpdater: Add $FALL-THROUGH$ comment to suppress false-positive warning

The fall-through is intentional.  Add the comment to prevent
Eclipse from warning about it.

Change-Id: I53c4a2686a4ca752bb5ad76169de34a929ac1267
diff --git a/src/main/java/com/google/gerrit/server/util/RefUpdater.java b/src/main/java/com/google/gerrit/server/util/RefUpdater.java
index ded742d..5a94722 100644
--- a/src/main/java/com/google/gerrit/server/util/RefUpdater.java
+++ b/src/main/java/com/google/gerrit/server/util/RefUpdater.java
@@ -177,6 +177,7 @@
           if (!delete && !args.isForceUpdate) {
             throw new IOException(result.name());
           }
+          // $FALL-THROUGH$
         case FAST_FORWARD:
         case NEW:
         case NO_CHANGE: