Add attributes: project::Branch(...) and project::DefaultBranch

[kondziu]

People often want to look at a project's just from the perspective of what's in the default branch (aka main or master branch).

Create an attributes project::Branch(x) and project::DefaultBranch which would return a new object that represented a branch. This object would have its own set of attributes, including branch::Commits.

This is an extension of an existing attribute: project::Heads, but one that would be more useful in the DSL.