cibtool trigger created a “ref changed” event without a base commit. This meant that the rest of the CI broker couldn’t construct a list of commits since the base commit, which meant it could not create a Request::Trigger message to the CI adapter. This meant that cibtool trigger was useless. Fix that by making sure the base commit is added: either the parent commit of the what is specified with --commit or that commit itself, if there is no parent commit.

Request::commit has always been fallible, but previously it has not returned a Result, and instead if anything went wrong, it panicked. Panics are not good, so change the method to return a Result instead. That’s a breaking change: callers will now have to handle the result.