RFR: git-pr: show commit messages

[Erik Helin]

Hi all,

please review this patch that adds commit messages to webrevs (if applicable).
The current `MetadataFormatter` is a bit bare-bones, I plan to enhance it in
future patches. I had to implement a few overloads of `commitMetadata` to
support this feature, hence the changes to `vcs`.

Testing:
- Added unit tests for `commitMetadata`
- Manual testing of `git-webrev` for both Git and Mercurial repositories

Thanks,
Erik

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Commits:
 - 33a8ded3: git-pr: show commit messages

Changes: https://git.openjdk.java.net/skara/pull/441/files
 Webrev: https://webrevs.openjdk.java.net/skara/441/webrev.00
  Stats: 329 lines in 11 files changed: 311 ins; 2 del; 16 mod
  Patch: https://git.openjdk.java.net/skara/pull/441.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/441/head:pull/441

PR: https://git.openjdk.java.net/skara/pull/441