RFR: 276: Merge style PRs should generate "merge" webrevs

[Robin Westberg]

Hi all,

Please review this change that for merge style PRs generate webrevs only containing the changes done in the merge
commit.

Note that this is only a partial implementation, such webrevs should also be generated if there are further commits
pushed to the PR.

Best regards,
Robin

-------------

Commit messages:
 - Initial implementation

Changes: https://git.openjdk.java.net/skara/pull/501/files
 Webrev: https://webrevs.openjdk.java.net/skara/501/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-276
  Stats: 110 lines in 3 files changed: 107 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/501.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/501/head:pull/501

PR: https://git.openjdk.java.net/skara/pull/501