Introduce a checkNotNull helper

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