Support Git implicit file:// URIs

Summary: @s reported an issue with implicit file:// URIs in Git, see P270.
Recognize and handle URIs in this format. For URIs we don't understand, raise an
exception.

Test Plan:

• Added failing tests.
• Fixed code.
• Tests pass.

Reviewers: btrahan, jungejason, s

Reviewed By: s

CC: aran, epriestley, s

Differential Revision: https://secure.phabricator.com/D1362