Use passthru to run git fetch in arc land so password prompts work

Summary:
Fixes T10314. In arc land, we currently run git fetch as a subprocess.

However, this can prevent password prompts (when fetching over HTTP with basic authentication) from working properly.

Instead, use passthru to redirect stdin/stdout to the subprocess so the user can type their password.

This adds a little extra clutter to the arc land output but I think that's OK.

Test Plan: See T10314, @maxie confirmed this fixes the issue.

Reviewers: chad

Reviewed By: chad

Subscribers: maxie

Maniphest Tasks: T10314

Differential Revision: https://secure.phabricator.com/D15233