Add support for path arguments via __ parameter

[ncoop57]

Adds ability to pass path arguments after the -- separator in git
commands.

Usage:

g.log('--oneline', __=['file1.py', 'file2.py'])
# → git log --oneline -- file1.py file2.py

This enables filtering git commands by specific files/paths using the standard git pathspec separator.

[jph00]

Smart! Can you add something to index.ipynb mentioning that too?