Introduce Repository.Refs.Head
Fix Lib/.gitattributes file
Add NameConflictException as wrapper for EEXIST native exception
Rename AmbiguousException as AmbiguousSpecificationException
Add InvalidSpecificationException as wrapper for EINVALIDSPEC native exception
Don't try to negate in an attributes file

This is not supported, and newer versions of git will outright refuse
to work with a tree that contains one.

This should keep the intent of the negation it's replacing.