FCPs proposed by the addition of labels to an issue with a previously-ignored command

[anp]

An example:

1. @nikomatsakis asks for a merge in https://github.com/rust-lang/compiler-team/issues/279#issuecomment-646049311
2. rfcbot ignores because no T-* labels
3. @nikomatsakis adds T-compiler in https://github.com/rust-lang/compiler-team/issues/279#event-3458364669
4. rfcbot continues to do nothing, presumably because we only check if a comment is an fcp start when the comment changes, not the issue?
5. @nikomatsakis invokes again in https://github.com/rust-lang/compiler-team/issues/279#issuecomment-646702640 and it works

Ideally, rfcbot could detect the state change in (3) as representing a proposal. I'm not sure it's a good idea to do that without also making sure that modifying team labels mid-FCP works correctly. If that's not feasible, it would at least be good for rfcbot to post a comment when (3) happens.

[nikomatsakis]

If that's not feasible, it would at least be good for rfcbot to post a comment when (3) happens.

This would be great I think and totally sufficient

[nikomatsakis]

Let me re-phrase: I think posting a comment is probably the most important thing and something that it ought to do regardless. That comment should explain what other steps (if any) are required.