Format @Nullable on return value as method annotation

Format @Nullable on return value as method annotation

For example, instead of using

	public @Nullable String getMyFavoriteString() { ... }

use

	@Nullable
	public String getMyFavoriteString() { ... }

This makes the style more consistent (the existing JGit code base
tends to lean toward the second style) and makes the source code
better reflect how the annotation is parsed, as a METHOD annotation.

Longer term, we should switch to a TYPE_USE annotation and switch to
the first style.

Noticed using a style checker that follows
https://google.github.io/styleguide/javaguide.html#s4.8.5-annotations

Change-Id: I07f4e67cc149fb8007f696a4663e10d4bfc57e3a
Reported-by: Ivan Frade <ifrade@google.com>
Signed-off-by: Jonathan Nieder <jrn@google.com>