Refactor Git.{AutoInterrupt,CatFileContentStream} nesting

This makes `Git.AutoInterrupt` and `Git.CatFileContentStream`
transparent aliases to top-level nonpublic `_AutoInterrupt` and
`_CatFileContentStream` classes in the `cmd` module.

This does not change the "public" interface. It also does not
change metadata relevant to documentation: the `__name__` and
`__qualname__` attributes are set explicitly to the values they had
before when these classes were defined nested, so that Sphinx
continues to document them (and to do so in full) in `Git` and as
`Git.AutoInterrupt` and `Git.CatFileContentStream`.

The purpose of this is to increase readability. The `Git` class is
big and complex, with a number of long members and various forms of
nesting. Since these two classes can be understood even without
reading the code of the `Git` class, moving the definitions out of
the `Git` class into top-level nonpublic classes will hopefully
increase readability and help with maintenance.

Author: EliahKagan