PackFileSnapshot: Resolve warnings by using "equals" instead of "=="

Errorprone raises the following warning: "[ReferenceEquality] Comparison
using reference equality instead of value equality"

Added Equality#isSameInstance util method to be reused in similar cases.

Change-Id: Ifc9885c9806dcafa1c5ee1351d3095bf1517dbf5

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackFileSnapshot.java

@@ -15,6 +15,7 @@
 
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.util.Equality;
 
 class PackFileSnapshot extends FileSnapshot {
 
@@ -61,7 +62,8 @@ public boolean isModified(File packFile) {
 	}
 
 	boolean isChecksumChanged(File packFile) {
-		return wasChecksumChanged = checksum != MISSING_CHECKSUM
+		return wasChecksumChanged = !Equality.isSameInstance(checksum,
+				MISSING_CHECKSUM)
 				&& !checksum.equals(readChecksum(packFile));
 	}
 

org.eclipse.jgit/src/org/eclipse/jgit/util/Equality.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2022, Fabio Ponciroli <ponch78@gmail.com> and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+package org.eclipse.jgit.util;
+
+/**
+ * Equality utilities.
+ *
+ * @since: 6.2
+ */
+public class Equality {
+
+    /**
+     * Compare by reference
+     *
+     * @param a
+     *            First object to compare
+     * @param b
+     *            Second object to compare
+     * @return {@code true} if the objects are identical, {@code false}
+     *         otherwise
+     *
+     * @since 6.2
+     */
+    @SuppressWarnings("ReferenceEquality")
+    public static <T> boolean isSameInstance(T a, T b) {
+        return a == b;
+    }
+}