is_dirty supports path. Fixes #482.

- add a second line to commit messages with the "BAD MESSAGE" text
- read in the log and confirm that the seond line is not in the log file

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

That way, we will not only get the summary line
contained in the blame, but fetch the full message.
This is more costly than the previous implementation
allowed it to be, but being less surprising/correct
certainly is the preferred behaviour here.

Fixes #485

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

It usually fails on branches, which doesn't help
assessing PRs.

Adding support for git remote set-url/get-url API to Remote

Don't choke on (legitimately) invalidly encoded Unicode paths