[skara] RFR: pr: add /clean command

[Erik Helin]

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

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Commit messages:
 - pr: add /clean command

Changes: https://git.openjdk.java.net/skara/pull/1024/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1024&range=00
  Stats: 400 lines in 5 files changed: 398 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/1024.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1024/head:pull/1024

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