RFR: cli: add debug command commit-comments

[Erik Helin]

Hi all,

please review this small patch that adds a utility debug command, `git skara debug commit-comments`, that lists recent commit comments for a given remote repository.

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - cli: add debug command commit-comments

Changes: https://git.openjdk.java.net/skara/pull/1013/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1013&range=00
  Stats: 206 lines in 2 files changed: 205 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1013.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1013/head:pull/1013

PR: https://git.openjdk.java.net/skara/pull/1013