Don't fatal when generating patch emails for diffs with binaries

Summary:
When Phabricator is configured to generate patch email, we'll fatal if the patch contains binaries and is generating to Git because ArcanistBundle can't load the binary data. Provide a callback to load the data. See D2174.

(This may cause us to generate absolutely enormous emails, but you get what you asked for...)

Test Plan: Created a diff with an image under "send git patches" email configuration.

Reviewers: Makinde, btrahan, vrana, jungejason

Reviewed By: Makinde

CC: aran

Differential Revision: https://secure.phabricator.com/D2175