Fix DiffusionGitBrowseQuery to parse with "git config -l -f" instead of PHP ini parser

Summary: See discussion on rPc0aac8267dda74664acac5c93d9aeb5a0f9c4564.

Test Plan: Looked at externals/, got a correctly-behaving link.

Reviewers: vrana, davidreuss, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2317