RFR: Proper selection of merge commit to squash commits onto for merge style PRs

[Robin Westberg]

Hi all,

Please review this change that properly selects the merge commit to squash eventual further commits onto in a
merge-style PR.

Best regards,
Robin

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Commit messages:
 - Proper selection of merge commit to squash commits onto for merge style PRs

Changes: https://git.openjdk.java.net/skara/pull/552/files
 Webrev: https://webrevs.openjdk.java.net/skara/552/webrev.00
  Stats: 135 lines in 3 files changed: 115 ins; 1 del; 19 mod
  Patch: https://git.openjdk.java.net/skara/pull/552.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/552/head:pull/552

PR: https://git.openjdk.java.net/skara/pull/552