Add a JGit-internal Nullable type

Commit 847b3d1 enabled annotation-based NPE analysis to JGit.

In so doing, it introduced a new dependency on the org.eclipse.jdt that
is undesirable. Follow Gerrit's lead by adding an internal Nullable type
(see
https://gerrit.googlesource.com/gerrit/+/stable-2.11/gerrit-common/src/main/java/com/google/gerrit/common/Nullable.java).

The javax.annotations.Nullable class uses a retention policy of RUNTIME,
whereas the org.eclipse.jdt.annotation.Nullable class used a policy of
CLASS. Since I'm not aware of tools that make use of the annotation at
runtime I chose the CLASS policy. If in the future there is a benefit to
retaining the annotations in the running binary we can update this
class.

Change-Id: I63dc8f9a6dc46b517cbde211bb5e2f8521a54d04
Signed-off-by: Terry Parker <tparker@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/annotations/Nullable.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2015, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.annotations;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.LOCAL_VARIABLE;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.PARAMETER;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * JGit's replacement for the {@code javax.annotations.Nullable}.
+ * <p>
+ * Denotes that a local variable, parameter, field, method return value can be
+ * {@code null}.
+ */
+@Documented
+@Retention(RetentionPolicy.CLASS)
+@Target({ FIELD, METHOD, PARAMETER, LOCAL_VARIABLE })
+public @interface Nullable {
+	// marker annotation with no members
+}