RFR: 685: Show a warning if PR author's full name does not match commit author's full name

[Erik Helin]

Hi all,

please review this patch that adds an error if the author of the pull request does not have an OpenJDK username, has a
full name set for their GitHub account and the `HEAD` commit for the pull request uses another full name.

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

Thanks,
Erik

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Commit messages:
 - 685

Changes: https://git.openjdk.java.net/skara/pull/840/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=840&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-685
  Stats: 64 lines in 5 files changed: 52 ins; 3 del; 9 mod
  Patch: https://git.openjdk.java.net/skara/pull/840.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/840/head:pull/840

PR: https://git.openjdk.java.net/skara/pull/840