RFR: 684: Pull request command parser parses too much

[Erik Helin]

Hi all,

please review this patch that ensures that `extractCommands` in `CommandWorkItem` only parses the parts of the PR body
that the user has written (i.e. scrub the automatic message from the bots).

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

Thanks,
Erik

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Commit messages:
 - 684

Changes: https://git.openjdk.java.net/skara/pull/825/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=825&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-684
  Stats: 50 lines in 3 files changed: 47 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/825.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/825/head:pull/825

PR: https://git.openjdk.java.net/skara/pull/825