RFR: 690: Merge commit message is not Merge jdk, but: Merge jdk

[Erik Helin]

Hi all,

please review this patch that makes it more clear what the expected and current commit messages are when showing the
error for `MergeMessageCheck`.

Thanks,
Erik

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

Commit messages:
 - 690
 - Merge jdk
 - Updated .jcheck/conf

Changes: https://git.openjdk.java.net/skara/pull/831/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=831&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-690
  Stats: 14 lines in 2 files changed: 12 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/831.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/831/head:pull/831

PR: https://git.openjdk.java.net/skara/pull/831