RFR: 362: Only amend merge commits that has the target branch as parent

[Robin Westberg]

Hi all,

Please review this change that avoids performing automatic rebasing of Merge PRs, as that doesn't work correctly in all
cases.

Best regards,
Robin

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Commit messages:
 - Only amend merge commits that has the target branch as parent

Changes: https://git.openjdk.java.net/skara/pull/584/files
 Webrev: https://webrevs.openjdk.java.net/skara/584/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-362
  Stats: 25 lines in 3 files changed: 18 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/584.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/584/head:pull/584

PR: https://git.openjdk.java.net/skara/pull/584