RFR: pr-bot: add contributors to summary

[Erik Helin]

Hi all,

please review this patch that adds information about contributors to the PR
body. Please note that we now have to run an extra pass with the `CheckWorkItem`
to make it possible to update the PR body based on the results from the
`CommandWorkItem`.

Testing:
- added a new unit test
- `make test` passes on Linux x64

Thanks,
Erik

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Commits:
 - 419e99e3: pr-bot: add contributors to summary

Changes: https://git.openjdk.java.net/skara/pull/429/files
 Webrev: https://webrevs.openjdk.java.net/skara/429/webrev.00
  Stats: 84 lines in 3 files changed: 84 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/429.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/429/head:pull/429

PR: https://git.openjdk.java.net/skara/pull/429