RFR: 1663: Mark integrated Pull Requests as properly merged in their repositories [v41]

[Julian Waters]

> Skara currently closes integrated Pull Requests, since the actual integration is done internally and then pushed to the repository separately. This makes every integrated request always look like it was closed to an outside observer, forcing the use of a special integrated flag to distinguish between integrated and closed, or rejected and closed. On top of that, it just looks awful in the interface all around (and is also not included in the accepted Pull Request count by GitHub for the committer, which is frustrating for newer contributors to a surprising extent). In the [willful absence of a reply from the GitHub team to allow marking a pull request as merged through a flag](https://github.com/orgs/community/discussions/12437), and [GitLab being unlikely to make a similar change either](https://gitlab.com/gitlab-org/gitlab/-/issues/16701), we can look at implementing this as an integration feature in Skara itself instead.
> 
> Every pull has a corresponding pre-integration branch created at the time it was made, and this special branch is marked for deletion when the pull is integrated, and initially it was thought to merge the Pull Request into these corresponding pr/ branches just before their deletion but that proved to present too many challenges to be viable. Instead a related approach of enhancing the Pull Request bots with their own special integration branches can be taken, as well as providing a flexible utility method for extracting the actual branch the Pull Request was integrated into, to address the concern of losing information on what the actual Pull Request target branch was.

Julian Waters has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 45 commits:

 - Merge remote-tracking branch 'upstream/master' into integration
 - Special Case in targetRef()
 - Just stick to the old method signature
 - Leave merging functionality for another enhancement
 - Merge remote-tracking branch 'upstream/master' into integration
 - Cleanup
 - GitLab should mirror the GitHub status getter
 - Field merged_at still exists even in unmerged Pull Requests
 - Minor commit comment change
 - Revert to more general repository
 - ... and 35 more: https://git.openjdk.org/skara/compare/98f997da...f8c8d446

-------------

Changes: https://git.openjdk.org/skara/pull/1409/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1409&range=40
  Stats: 143 lines in 6 files changed: 88 ins; 20 del; 35 mod
  Patch: https://git.openjdk.org/skara/pull/1409.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1409/head:pull/1409

PR: https://git.openjdk.org/skara/pull/1409