Ignore contents of `.gitignore`

[hugovk]

Context

We're going to start using sphinx-autobuild in python/devguide#1208 and are adding the PyCharm IDE directory (.idea) and virtualenv directory (venv) to prevent irrelevant changes from triggering a rebuild:

--re-ignore="/\.idea/|/venv/"

Proposal

It would be helpful if sphinx-autobuild read in the contents of any .gitignore and automatically ignored any files matching the contents.

This should be in addition to any files specified via command-line options.

Tasks and updates

No response