RFR: pr: separate commit and pr commands

[Erik Helin]

Hi all,

please review this pull request that properly separates pull request and commit commands at the source code level. The class `CommandWorkItem` is renamed to `PullRequestCommandWorkItem` and then I did a number of smaller follow-out refactorings from this renaming.

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - pr: separate commit and pr commands

Changes: https://git.openjdk.java.net/skara/pull/1038/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1038&range=00
  Stats: 89 lines in 8 files changed: 61 ins; 0 del; 28 mod
  Patch: https://git.openjdk.java.net/skara/pull/1038.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1038/head:pull/1038

PR: https://git.openjdk.java.net/skara/pull/1038