RFR: Improve handling of squash and rebase of PRs

[Robin Westberg]

Hi all,

Please review this change that improves how we perform squashing and rebasing before doing the final integration.
Instead of using squash / rebase, simply merge the latest state of the target branch. Use the final state (tree) to
create the commit, and specify the proper parents. This has the benefit of working for merge PRs as well.

Best regards,
Robin

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Commit messages:
 - Proper validation of merge sources
 - Initial implementation

Changes: https://git.openjdk.java.net/skara/pull/589/files
 Webrev: https://webrevs.openjdk.java.net/skara/589/webrev.00
  Stats: 437 lines in 12 files changed: 265 ins; 132 del; 40 mod
  Patch: https://git.openjdk.java.net/skara/pull/589.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/589/head:pull/589

PR: https://git.openjdk.java.net/skara/pull/589