tg-prev/tg-next: commands to explore dependencies

Two new commands to explore the dependencies of TopGit branches:

  a) tg prev [-i | -w] [NAME]
     outputs the dependencies of NAME

  b) tg next [-i | -w] [NAME]
     outputs branches which depends on NAME

Obviously, quilt next was the inspiration.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>