RFR: Allow merge PRs with an exact hash

[Erik Helin]

On Mon, 22 Feb 2021 09:35:52 GMT, Robin Westberg <rwestberg at openjdk.org> wrote:

> Make it possible to create a merge-style PR with commits that are not already present in another existing repository. In this case, the hash of the commit to be merged must be specified, to allow for proper rebase / squash of the merge commit itself.

Looks good overall, really nice solution �� Just two things I thought of:

- should we check that the the hash actually is an ancestor of the source branch? That check combined with the existing check that the hash is not an ancestor of the target branch would ensure that the hash only is in the source branch for the pull request. Would safe-guard against some extremely unlikely scenarios �� 
- can you make it possible via a bot configuration to restrict the users that can do merge style pull requests with a hash for a repository? I don't think all repositories are gonna want to allow everyone to use a `Merge <HASH>` kind of pull request.

Thanks!

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PR: https://git.openjdk.java.net/skara/pull/1026