Format @NonNull on return value as method annotation

Format @NonNull on return value as method annotation

For example, instead of using

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

use

	@NonNull
	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: I9b9fa08035d805ca660520f812a84d2f47eff507
Reported-by: Ivan Frade <ifrade@google.com>
Signed-off-by: Jonathan Nieder <jrn@google.com>