PatchListLoader: Use equals instead of == for comparison

In this case there is not actually a bug, because it's comparing two
RevCommit objects from the same RevWalk, which by design uses the same
instance every time a particular SHA-1 is referenced. However, this is
not obvious to reason about.

Change-Id: I19df77c04a661e5fcb2b0e021ceccfeac2d43673

gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListLoader.java

@@ -151,7 +151,7 @@ public class PatchListLoader implements Callable<PatchList> {
       }
 
       final boolean againstParent =
-          b.getParentCount() > 0 && b.getParent(0) == a;
+          b.getParentCount() > 0 && b.getParent(0).equals(a);
 
       RevCommit aCommit = a instanceof RevCommit ? (RevCommit) a : null;
       RevTree aTree = rw.parseTree(a);