RFR: cli: better hg support in git-jcheck

[Erik Helin]

Hi all,

please review this patch that adds better support for the `--mercurial` flag to
`git-jcheck`. Some newer (git specific) features will be disabled when
`--mercurial` is passed and errors will not be shown for missing email
addresses.

Testing:
- Manual testing of `git jcheck --mercurial` on Linux x64

Thanks,
Erik

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Commit messages:
 - cli: better hg support in git-jcheck

Changes: https://git.openjdk.java.net/skara/pull/528/files
 Webrev: https://webrevs.openjdk.java.net/skara/528/webrev.00
  Stats: 14 lines in 2 files changed: 4 ins; 1 del; 9 mod
  Patch: https://git.openjdk.java.net/skara/pull/528.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/528/head:pull/528

PR: https://git.openjdk.java.net/skara/pull/528