Moves "create diff/patch" logic out of the view and into the diffparser. This way the view just deals with "strings" rather than having to create diff fragments.

Moving the logic out of the view also means that the raw patch can be exposed in other places, e.g. we could expose the diff in an email (NOTE if this was done would need to be configurable so that normal users don't suffer the overhead of generating raw patches, e.g. lazy/deferred evaluation or a config option).

Related to old (svn) review http://reviews.reviewboard.org/r/598