ReceiveCommits: Use equals for comparison

In this case there is not actually a bug, as the RevCommits are coming
from the same RevWalk so the parent pointers are to the same
instances. However, this is non-obvious if you aren't familiar with
RevWalk, and ErrorProne flags it as well.

Change-Id: I5db4f662565dc8890abd746499cc3e46409e40f9
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java b/gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java
index 08be313..6edf243 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java
@@ -2275,7 +2275,7 @@
       return false;
     }
     for (int i = 0; i < a.getParentCount(); i++) {
-      if (a.getParent(i) != b.getParent(i)) {
+      if (!a.getParent(i).equals(b.getParent(i))) {
         return false;
       }
     }