Implement ObjectReader.new_reader!/1 and has.object?/2.

[scouten]

Changes in This Pull Request

• Add new_reader!/1 and has_object?/2 to Xgit.Lib.ObjectReader interface.
• Add a minimal implementation of WindowCursor.

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 notices.
• The Eclipse Distribution License header text is included in all new source files.
• 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.