Abstract raw diff rendering into DifferentialRawDiffRenderer

Test Plan:
Enable inline patches:

```
bin/config set metamta.differential.patch-format 'unified'
bin/config set metamta.differential.inline-patches 100000000
```

Create a new diff and confirm it renders correctly via email.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D7198