Use `git config --list -z` to support multi-line git aliases.
Use `git config --list -z` to support multi-line git aliases.

Summary: This is what `git help config` recommends.  And will help komalo.

Test Plan:
In the gitbigfile/ dir, ran
   python -c 'import util; print util.get_git_config()'
and the output looks like the config-dict.

Reviewers: benkomalo

Reviewed By: benkomalo

Differential Revision: https://phabricator.khanacademy.org/D27709

Khan/git-bigfile