Implement Xgit.Core.PersonIdent.

[scouten]

Changes in This Pull Request

Bring over PersonIdent module from jgit port. This struct describes the tuple of {name, email, time, timezone} as recorded for various git operations.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• There is test coverage for all changes.
• Any code ported from jgit maintains all existing copyright and license notices.
• If new files are ported from jgit, the path to the corresponding file(s) is included in the header comment.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment. n/a