RFR: Restrict merge PR commit titles if required by the jcheck configuration

[Robin Westberg]

If the jcheck configuration requires a certain commit message for merges, attempt to ensure that a merge PR result in a conforming message.

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Commit messages:
 - Restrict merge PR commit titles if required by the jcheck configuration

Changes: https://git.openjdk.java.net/skara/pull/949/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=949&range=00
  Stats: 126 lines in 2 files changed: 109 ins; 4 del; 13 mod
  Patch: https://git.openjdk.java.net/skara/pull/949.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/949/head:pull/949

PR: https://git.openjdk.java.net/skara/pull/949