RFR: pr: filter external commands in CommandWorkItem

[Erik Helin]

Hi all,

please review this patch that ensures that `CommandWorkItem` correctly filters out external commands when looking for
commands to process in a pull request.

Testing:
- [x] Added a new unit test
- [x] `make test` passes on Linux x64

Thanks,
Erik

-------------

Commit messages:
 - pr: filter external commands in CommandWorkItem

Changes: https://git.openjdk.java.net/skara/pull/695/files
 Webrev: https://webrevs.openjdk.java.net/skara/695/webrev.00
  Stats: 68 lines in 2 files changed: 50 ins; 11 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/695.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/695/head:pull/695

PR: https://git.openjdk.java.net/skara/pull/695