RFR: jcheck: allow pattern for merge commit messages

[Erik Helin]

Hi all,

please review this patch that allows for the merge message variable in the
.jcheck/conf to be a pattern. I added an additional unit test for this feature.

Testing:
- `make test` passes on Linux x64
- Added an additional unit test

Thanks,
Erik

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Commit messages:
 - jcheck: allow pattern for merge commit messages

Changes: https://git.openjdk.java.net/skara/pull/493/files
 Webrev: https://webrevs.openjdk.java.net/skara/493/webrev.00
  Stats: 16 lines in 2 files changed: 13 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/493.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/493/head:pull/493

PR: https://git.openjdk.java.net/skara/pull/493