Force delete

Adding support for `git branch -D` as a flag for Repository::deleteBranch and BranchCommand::delete.
Just verifying that the command here is formed properly, not actually executing it (as the lines below do).  I'm not sure this unit test SHOULD be executing the command, since that's covered by RepositoryTest::testDeleteBranch, but I left it alone because it's outside the scope of this ticket.