Accept/decline T-compiler backports from Zulip

[apiraino]

This patch adds a new Zulip command:
@triagebot backport <channel> <action> <PR>

Example:

• @triagebot backport stable accept 123456
• @triagebot backport beta decline 654321

This can be used to post on GitHub a comment like this one when T-compiler accepts/decline to backport a patch that fixes a regression.

Limitations:

• This command assumes that the PR is from rust-lang/rust, I am not aware of other git repos having this workflow. If we want, maybe in the future we can extend to other repos using our custom Zulip linkfiers
• There is no check on the PR number (so if someone mistypes, the comment will go elsewhere 😅 )

Tested on a custom Zulip instance.

TODO:

• update docs on the forge
• Could be interesting a shorter variant of this command (f.e. backport accept or even accept) to be used from #t-compiler/backports (topics contain all necessary info to build the command parameters)

[apiraino]

Note: for how the Message API works today, we can't pin exactly a single message so the link in the GitHub comment will be to the whole topic, which is a bit inconvenient but hopefully will be solved by Zulip. I'd rather wait on them for a correct fix rather us add hacks to make it work.

[apiraino]

I guess this patch went under the radar because I didn't request a review

r? triagebot

[Kobzol]

Sounds pretty useful!

View changes since this review

[Kobzol]

Could you please use an enum here for the channel and the verb, and either generate the parser with #[derive(clap::ValueEnum)], or write it manually with FromStr? That way we won't have to check for invalid data in the backport handler, and we'll get a nice error message and a hint (in case of misspelling) for free.