Use Objects.requireNonNull instead of a custom helper

This simplifies the API surface and makes JGit internals into less of
a custom Java dialect.

Change-Id: Idbb7d4d1037c5336341088385b8e0a59c8b4c952

org.eclipse.jgit/src/org/eclipse/jgit/api/Git.java

  */
 package org.eclipse.jgit.api;
 
-import static org.eclipse.jgit.lib.Constants.checkNotNull;
+import static java.util.Objects.requireNonNull;
 
 import java.io.File;
 import java.io.IOException;
 	}
 
 	Git(Repository repo, boolean closeRepo) {
-		this.repo = checkNotNull(repo);
+		this.repo = requireNonNull(repo);
 		this.closeRepo = closeRepo;
 	}
 

org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java

 	 */
 	public static final String ATTR_BUILTIN_BINARY_MERGER = "binary"; //$NON-NLS-1$
 
-	/**
-	 * Null checker for a {@code @NonNull} parameter.
-	 *
-	 * <p>This is a briefer equivalent to
-	 * <pre>
-	 * if (arg == null) {
-	 *   throw new NullPointerException();
-	 * }
-	 * </pre>
-	 * with the added benefit that it does not trigger nullness warnings when
-	 * {@code arg} is declared as {@code @NonNull}.
-	 *
-	 * @param arg a non-null object reference
-	 * @return arg
-	 * @throws NullPointerException if {@code arg} is null
-	 * @since 5.4
-	 */
-	public static <T> T checkNotNull(T arg) {
-		if (arg == null) {
-			throw new NullPointerException();
-		}
-		return arg;
-	}
-
 	/**
 	 * Create a new digest function for objects.
 	 *