RFR: Try a bit harder to generate incremental webrevs

[Robin Westberg]

Hi all,

Please review this change that makes the mailing list bridge bot try a little harder to generate an incremental webrev. If a pull request has been rebased or merged the latest changes from the target branch, it may be possible to exclude these and generate a clean incremental webrev. If so, we do that.

Best regards,
Robin

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Commits:
 - 9a37cbf2: Try a bit harder to generate incremental webrevs

Changes: https://git.openjdk.java.net/skara/pull/295/files
 Webrev: https://webrevs.openjdk.java.net/skara/295/webrev.00
  Stats: 105 lines in 4 files changed: 102 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/295.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/295/head:pull/295

PR: https://git.openjdk.java.net/skara/pull/295