RFR: Mark integrated Pull Requests as properly merged in GitHub repositories

[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. In the [willful absence of a reply from the GitHub team to allow marked a pull as merged through a flag](https://github.com/orgs/community/discussions/12437), and GitLab being unlikely to make a similar change either, a workaround with minimal impact can be used 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. Following the principle of waste not, want not, a change that is currently only implemented for GitHub repositories instead resets the pre-integratio
 n branch to the source ref of the pull request, then merges the request into it's own pre-integration branch before the actual deletion occurs, leaving minimal impact on most of Skara's infrastructure and with virtually no effect on already existing repositories.

- [x] Properly handle all dependencies of the pre-integration branch before it's modified irreversibly

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Commit messages:
 - Squash for neatness

Changes: https://git.openjdk.org/skara/pull/1409/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1409&range=00
  Stats: 101 lines in 7 files changed: 51 ins; 19 del; 31 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