Custom git binary support

Provide support for specifying a custom `git` executable. Since we need to ensure that our omnibussed `git` is executed without mucking around with my PATH, there is now a `@@git_binary` variable in `Git::Lib` which can be set via `Git::Lib.git_binary = '/path/to/git'`.
There are a couple of changes like this on the pipeline. It may be a good time to add a single configuration entry.

Thinking in something like:

    Git.config do |config|
      config.binary_path = 'path/to/binary'
      config.ssh_keys = 'path/to/key'
    end