RFR: jcheck: make merge committers configurable

[Erik Helin]

Hi all,

please review this patch that adds an additional configuration to the
"committers" check. We now support setting usernames that are allowed to be
committers of merge commits even though they are not part of the project. This
is useful for allowing automated merge commits to pass jcheck.

Testing:
- `make test` passes on Linux x64
- Added two new unit tests

Thanks,
Erik

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Commit messages:
 - jcheck: make merge committers configurable

Changes: https://git.openjdk.java.net/skara/pull/494/files
 Webrev: https://webrevs.openjdk.java.net/skara/494/webrev.00
  Stats: 67 lines in 3 files changed: 61 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/494.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/494/head:pull/494

PR: https://git.openjdk.java.net/skara/pull/494