[skara] RFR: pr: only act on commit comments on known branches

[Erik Helin]

Hi all,

please review this patch that makes the `CommitCommentsWorkItem` filter out commit comments that aren't made on commits on a known branch in the remote repository. Some forges can return comments on commits in pull requests, which aren't comments we want to act upon (commit commands are only for integrated commits on a known branch).

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

Thanks,
Erik

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Commit messages:
 - pr: only act on commit comments on known branches

Changes: https://git.openjdk.java.net/skara/pull/1034/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1034&range=00
  Stats: 94 lines in 3 files changed: 83 ins; 7 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/1034.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1034/head:pull/1034

PR: https://git.openjdk.java.net/skara/pull/1034