RFR: Restrict merge PR commit titles if required by the jcheck configuration [v3]

[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.

Robin Westberg has updated the pull request incrementally with one additional commit since the last revision:

  Fix failing test

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/949/files
  - new: https://git.openjdk.java.net/skara/pull/949/files/89ce463e..de347035

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=949&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=949&range=01-02

  Stats: 20 lines in 1 file changed: 5 ins; 12 del; 3 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