RFR: 1663: Mark integrated Pull Requests as properly merged in GitHub repositories [v2]

[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, 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-integration 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 (and it also keeps the aesthetically pleasing integrated flag!).
> 
> - [x] Properly handle all dependencies of the pre-integration branch before it's modified irreversibly
> - [ ] Change test code to match modified integration logic (if required)

Julian Waters has updated the pull request incrementally with one additional commit since the last revision:

  Revert changes to PreIntegrations and try something stupid instead

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

Changes:
  - all: https://git.openjdk.org/skara/pull/1409/files
  - new: https://git.openjdk.org/skara/pull/1409/files/078b4f77..8ca08323

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1409&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1409&range=00-01

  Stats: 17 lines in 2 files changed: 2 ins; 14 del; 1 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