Incorrect labels after "bors r- retry"

[RalfJung]

Doing bors r- retry (e.g. because we realized a PR would fail but is already being tested, so we want it to be removed from the queue and the active job to be stopped) leaves the PR with incorrect labels: the PR is still labeled "waiting for bors". This also happen when first doing "bors r-" and then in another comment doing "bors retry". See rust-lang/rust#66995 (comment) for an example.

I guess "bors retry" fails to detect that the PR is not actually queued any more, and thus incorrectly sets the "waiting for bors" label.

[flip1995]

Since this is something that has to be done fairly often (in tools or tool updates at least), maybe it would be worth adding a bors abort command for that.