RFR: 342: Pull request bot should warn on merge commits with non-target branch parent

[Robin Westberg]

Hi all,

Please review this change that checks if a PR contains merges from outside the current project, and suggests using a
merge-style PR.

Best regards,
Robin

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Commit messages:
 - Detect when a merge PR could be appropriate

Changes: https://git.openjdk.java.net/skara/pull/679/files
 Webrev: https://webrevs.openjdk.java.net/skara/679/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-342
  Stats: 103 lines in 3 files changed: 103 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/679.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/679/head:pull/679

PR: https://git.openjdk.java.net/skara/pull/679