Simplify autoCloseCommits() handling of exact SHA-1 matches

Ref objects inside of the byCommit map cannot be null.
The ref != null test is always true, and can be dropped.

While we are cleaning up this code, inline the Set<Ref>
that is used only for the loop control.

Change-Id: I8fc9a62672b94203ababf8f5b529ded727cf700a
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 84d04f8..2b3cc57 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
@@ -2391,15 +2391,12 @@
       for (RevCommit c; (c = rw.next()) != null;) {
         rw.parseBody(c);
 
-        final Set<Ref> refs = byCommit.get(c.copy());
-        for (Ref ref : refs) {
-          if (ref != null) {
-            Change.Key closedChange =
-                closeChange(cmd, PatchSet.Id.fromRef(ref.getName()), c);
-            closeProgress.update(1);
-            if (closedChange != null) {
-              byKey.remove(closedChange);
-            }
+        for (Ref ref : byCommit.get(c.copy())) {
+          Change.Key closedChange =
+              closeChange(cmd, PatchSet.Id.fromRef(ref.getName()), c);
+          closeProgress.update(1);
+          if (closedChange != null) {
+            byKey.remove(closedChange);
           }
         }