Use HTTPEngineExtension proxy for git HTTP operations

Summary: Ref T10227. When we perform git http operations (fetch, mirror) check if we should use a proxy; if we should, set http_proxy or https_proxy in the environment to make git have curl use it.

Test Plan:

• Configured a proxy extension to run stuff through a local instance of Charles.
• Ran repository pull and repository mirror.
• Saw git HTTP requests route through the proxy.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10227

Differential Revision: https://secure.phabricator.com/D16092