RFR: pr: add /clean command

[Robin Westberg]

On Fri, 19 Feb 2021 11:33:28 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that adds the `/clean` pull request command for marking a backport pull request as clean. The command can be used by project Committers who determine that a backport is clean (even though the bot doesn't think so). This can for example be for simple things like the backport changing the copyright year, which the bot will not consider clean, but a human might.
> 
> I have also added the `--clean` flag to `git pr set` for those that prefer working from the command-line.
> 
> Testing:
> - [x] Added a number of unit tests
> - [x] `make test` passes on Linux x64
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/1024