Probot Client should be able to update its own comment

[ckerr]

Right now the probot client's workflow for bisect is

1. start bisect
2. wait for bisect to finish
3. give the results in a new comment

Instead, it should be:

1. add a comment saying that the bisect is beginning
2. start bisect
3. wait for bisect to finish
4. give the results by updating the comment created in step 1

The bot should be refactored in a way s.t. it should not care whether the first comment exists yet or not, e.g. GithubClient.setBugbotComment(markdown: string) (or whatever function name) so that the comment is created if it doesn't exist, or updated if it does exist.

We will need this functionality for the matrix as well, but adding it for the existing bisect feature means it could be developed now before the matrix code is ready.