Introduce a checkNotNull helper

When using @NonNull annotations in new code, if I write

	public void setFrobber(@NonNull frobber) {
		this.frobber = frobber;
	}

then consumers of the JGit library that do not have nullness checking
enabled can easily pass in null by mistake.  On the other hand, if I
write

	public void setFrobber(@NonNull frobber) {
		if (frobber == null) {
			throw new NullPointerException();
		}
		this.frobber = frobber;
	}

then Eclipse JDT complains:

	Null comparison always yields false: The variable frobber is specified as @NonNull

Add a checkNotNull helper that offers the best of both worlds:

	public void setFrobber(@NonNull frobber) {
		this.frobber = checkNotNull(frobber);
	}

Briefer code, null check is intact, and no warning.

Inspired by Guava's com.google.common.base.Preconditions.checkNotNull.

Change-Id: If59588d13a1119e899657ed2296931ea18ed0e2a

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

@@ -43,6 +43,8 @@
  */
 package org.eclipse.jgit.api;
 
+import static org.eclipse.jgit.lib.Constants.checkNotNull;
+
 import java.io.File;
 import java.io.IOException;
 
@@ -220,9 +222,7 @@ public class Git implements AutoCloseable {
 	}
 
 	Git(Repository repo, boolean closeRepo) {
-		if (repo == null)
-			throw new NullPointerException();
-		this.repo = repo;
+		this.repo = checkNotNull(repo);
 		this.closeRepo = closeRepo;
 	}
 

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

@@ -58,7 +58,7 @@ import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.util.MutableInteger;
 
 /**
- * Misc. constants used throughout JGit.
+ * Misc. constants and helpers used throughout JGit.
  */
 @SuppressWarnings("nls")
 public final class Constants {
@@ -465,6 +465,30 @@ public final class Constants {
 	 */
 	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.
 	 *