It's not optimal, as we can now return one of two types
which are only compatible in the most basic ways.

However, it is better than before, I presume.

Fixes #510

It's entirely untested if this repo still does the right thing,
but I'd think it does.

Fixes #504

As TagObjects can point to other TagObjects, we need
to keep going in order to resolve the final commit.

Fixes #503

That way, we do not have to figure the change type out by
examining the diff object.

It's implemented in a way that should yield more desireable results
as we keep the change-type that git is providing us with.

Fixes #493

Previously, the following fields on Diff instances were assumed to be
passed in as unicode strings:

  - `a_path`
  - `b_path`
  - `rename_from`
  - `rename_to`

However, since Git natively records paths as bytes, these may
potentially not have a valid unicode representation.

This patch changes the Diff instance to instead take the following
equivalent fields that should be raw bytes instead:

  - `a_rawpath`
  - `b_rawpath`
  - `raw_rename_from`
  - `raw_rename_to`

NOTE ON BACKWARD COMPATIBILITY:
The original `a_path`, `b_path`, etc. fields are still available as
properties (rather than slots).  These properties now dynamically decode
the raw bytes into a unicode string (performing the potentially
destructive operation of replacing invalid unicode chars by "�"'s).
This means that all code using Diffs should remain backward compatible.
The only exception is when people would manually construct Diff
instances by calling the constructor directly, in which case they should
now pass in bytes rather than unicode strings.

See also the discussion on
https://github.com/gitpython-developers/GitPython/pull/467

Relates to #446