Simplify Git diff API

[jonasehrlich]

The API now only provides the following data:

struct Diff {
    /// Patch between old and new
    patch: String,
    /// Stats of the diff
    stats: DiffStats,
    /// Map of old source paths to the old content
    old_sources: hash_map::HashMap<Path, FileContent>,
}

This is easier to handle on the client side.

[jonasehrlich]

⚠️ Requires #93 to be merged

[TobTheRock]

Github really lacks merge dependencies, some comments on #95 apply actually to this change..