RFR: pr: only warn about differing author names

[Erik Helin]

Hi all,

please review this patch that slightly updates the pull request full name checking to only write a warning instead of
failing jcheck. This patch also provides clear instructions in the warning comment for how to create a new commit with
the desired full name.

Testing:
- [x] Updated the existing unit test
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - pr: only warn about differing author names

Changes: https://git.openjdk.java.net/skara/pull/849/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=849&range=00
  Stats: 89 lines in 8 files changed: 71 ins; 10 del; 8 mod
  Patch: https://git.openjdk.java.net/skara/pull/849.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/849/head:pull/849

PR: https://git.openjdk.java.net/skara/pull/849