Create the "tg annihilate" command.

This annihilates a TopGit branch, making it disappear from the output
of "tg summary".  All changes made on it are lost.

Signed-off-by: Per Cederqvist <ceder@lysator.liu.se>
Signed-off-by: Robin Green <greenrd@greenrd.org>
tg-annihilate: Pass --no-verify when committing.

This is needed in order to inhibit TopGit's pre-commit hook to run,
which would bark upon missing .top* files.

Signed-off-by: Robin Green <greenrd@greenrd.org>