Require latest Git::PurePerl

As 0.48 and older blow up on 5.18+.
Convert Object to M::S

For just the Object class at least. It was a pretty painless transition
so will apply this elsewhere.
Convert Repository to M::S

Sort out the syntax too. Only concession I've had to make to M::S is
fully qualified types, it seems it only understands the shorthand for
the standard types and not those that have been imported.
Convert Commit to M::S

Just like what happened to Repository.
Convert the remaining Object classes to M::S

Same deal as before.
Convert Util classes to M::S

Which was the last of the library code to have MX::D to be excised. Now
to root it out entirely.
Drop remaining Gitalist usage

And therefore the MX::D dependencies. The code now passes 'make test'
cleanly.