RFR: 1584: Require re-review of a PR if the target branch changes [v2]

[Erik Joelsson]

> This change improves reviewer handling when the target branch of a PR is changed. Currently this only invalidates a review on GitHub, but not on GitLab. With this change the behavior is made uniform. In addition to this, the following new behavior is added.
> 
> 1. When the target branch of a PR is changed, any existing reviews in the Reviewers list will be flagged as "Re-review required (review applies to pull request targeting <branch>)" to make what happened clearer.
> 2. If the target branch of a PR is changed back to a previously targeted branch, any reviews made against that branch may now become valid again (if all other requirements are met).
> 
> Note that changes to and from pr/X branches are ignored, just like before.
> 
> PullRequest::reviews is no longer specifying the order of reviews returned. This order was actually not consistent between implementations and I would rather we use explicit sorts when needed instead of assuming an order.
> 
> I'm not really happy with the placement of the new static method `calculateReviewTargetRefs`, but I couldn't really find a better location, and I really wanted to share this between different `PullRequest` implementations. I think the correct solution would be to introduce an `AbstractPullRequest` common super class, but I didn't want to do this for a single method.
> 
> I have verified this with the new and modified tests, as well as by running against the playground repo(s).

Erik Joelsson has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge branch 'master' into SKARA-1584-rereview-targetref
 - SKARA-1584

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Changes: https://git.openjdk.org/skara/pull/1383/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1383&range=01
  Stats: 299 lines in 13 files changed: 246 ins; 13 del; 40 mod
  Patch: https://git.openjdk.org/skara/pull/1383.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1383/head:pull/1383

PR: https://git.openjdk.org/skara/pull/1383