Implement rollup commands

[Kobzol]

homu has commands for marking a PR as being rollup=[never/iffy/maybe/always].

Bors can't currently do merges, but we could implement parsing of the command, checking of permissions etc., storing of the rollup state in the DB for a specific PR etc.

It would be great to check out how homu does this.

[geetanshjuneja]

Hi I would like work on this. Also can I claim more than one issue?

[geetanshjuneja]

@rustbot claim

Ohh rustbot isn't enabled here

[rustbot]

Error: This repository is not enabled to use triagebot.
Add a triagebot.toml in the root of the default branch to enable it.

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.

[Kobzol]

I don't think it's that important to claim stuff :) Yeah, we don't use triagebot here.

[geetanshjuneja]

if in case rollup isn't specified in approval default would be "maybe"?

[Kobzol]

Yes, that's the default behavior.