Nullable: Switch to TYPE_USE

Nullable: Switch to TYPE_USE

Since JGit now requires Java 8, we can switch to TYPE_USE instead
of explicitly specifying the target type.

Some of the existing uses of Nullable need to be reworked slightly
as described in [1] to prevent the compilation error:

  scoping construct cannot be annotated with type-use annotation

[1] https://stackoverflow.com/a/21385939/381622

Change-Id: Idba48f67a09353b5237685996ce828c8ca398168
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>