[findbugs] Fix TrackingId.equals()

Comparing tr.key to itself is always true which yields wrong results
for equals().

Change-Id: I284d2c651e8ab23aa81d71594feefc5148638840
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/TrackingId.java

@@ -144,7 +144,7 @@ public final class TrackingId {
   public boolean equals(final Object obj) {
     if (obj instanceof TrackingId) {
       final TrackingId tr = (TrackingId) obj;
-      return tr.key.equals(tr.key);
+      return key.equals(tr.key);
     }
     return false;
   }