Xgit.Repository: Implement symbolic references.

[scouten]

Changes in This Pull Request

References of the form ref: (ref name) are now properly followed when invoking get_ref/3 or put_ref/3.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• There is test coverage for all changes.
• All cases where a literal value is returned use the cover macro to force code coverage.
• Any code ported from jgit maintains all existing copyright and license notices. n/a
• If new files are ported from jgit, the path to the corresponding file(s) is included in the header comment. n/a
• 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

[scouten]

Closes #228.