[skara] RFR: pr: fetch commit comments in work item

[Erik Helin]

Hi all,

please review this patch that fetches recent commit comments in a separate `WorkItem` for the `PullRequestBot`. This is to ensure that:

- possible exceptions thrown when fetching recent commit comments do not impact other `WorkItem`s generated by `PullRequestBot.getPeriodicItems`
- other `WorkItem`s can continue to make progress if the call to `recentCommitComments` takes a long time

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

Thanks,
Erik

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Commit messages:
 - pr: fetch commit comments in work item

Changes: https://git.openjdk.java.net/skara/pull/1031/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1031&range=00
  Stats: 88 lines in 3 files changed: 67 ins; 17 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/1031.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1031/head:pull/1031

PR: https://git.openjdk.java.net/skara/pull/1031