RFR: jcheck: conditionally check backport reviewers

[Robin Westberg]

On Wed, 14 Oct 2020 14:30:11 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that makes it possible to configure whether reviewers are required or not for backport
> commits. The `reviewers` check now supports the configuration `backports=check` or `backports=ignore` with
> `backports=ignore` being the default. We will still check for invalid reviewers and self-reviews, it is just the
> "number of reviewers" that can be disabled for backport commits.  Testing:
> - [x] Added three new 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/884