[llvm] [GitHub] Add workflows to manage merging of PRs from authors without commit access (PR #124910)

[Renato Golin via llvm-commits]

rengolin wrote:

> * I considered other things here, but comments is the only thing we can rely on a new contributor being able to add.
> * I have handled the case where the author alreay knows the process and writes the magic comment ahead of time. I think it's valid to allow this as long as they're not obviously doing it to skip out on writing a decent PR description. Reviewers should be able to judge this.

This is reasonable. Many Github repos have actions in response to keywords in comments (run tests, benchmarks, trigger external CI, etc).

> There is a corner case where you get 1 approval, and go through this process, but another reviewer vetos that approval. In this case, the label can be manually added and the bot comments and the user's magic comment removed, reseting the situation.

Manually adding requires authors and/or reviewers to know they have to do it, and documentation is hard in this case because it's "elsewhere".

Another corner case is removing the tag and warning me but I miss the ping (lost in emails, crash browser) and then the state is lost.

How about we have two tags: `no-commit-access` and `ready-to-merge`?

Workflow:
```
New PR from read-only user -> sets `no-commit-access`
(optional) Author comments "ready to merge" -> sets `ready-to-merge`
(optional) Reviewer request changes

---- WHILE (has_review) {
    Branch is updated -> removes `ready-to-merge`
    (optional) Author comments "ready to merge" -> sets `ready-to-merge`
    (optional) Reviewer request changes
---- }

---- IF (`ready-to-merge`) {
    Reviewer approves and merges
---- ELSE
    Reviewer approves
    Author comments "ready to merge" -> sets `ready-to-merge`
    Any read-write user can merge
---- }
```

https://github.com/llvm/llvm-project/pull/124910