ReceiveCommits: Use checkNotNull instead of checkState

To prevent false-positive warning from FindBugs about null value.

Change-Id: I6c0b48275631ccfd9d7d0c1d16932a19d9613bbe
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 66637fb..450c198 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
@@ -1824,7 +1824,7 @@
       bySha.put(r.newCommitId, r.notes.getChange());
     }
     Change tipChange = bySha.get(magicBranch.cmd.getNewId());
-    checkState(tipChange != null,
+    checkNotNull(tipChange,
         "tip of push does not correspond to a change; found these changes: %s",
         bySha);
     try (MergeOp op  = mergeOpProvider.get()) {