a8n: Align API for previews and external changesets

Created by: eseliger

I know I signed this off earlier, but it turns out there are quite a few places where ChangesetPlan and ExternalChangeset are used in the same context, but they currently are not very similar in their structure.

For example, in the WebApp we have to always check the __typename property before we can use the diff:

const diffStat = campaign
        ? campaign.__typename === 'CampaignPlan'
            ? campaign.diff.fileDiffs.diffStat
            : campaign.fileDiffs.diffStat
        : null

current schema

type ChangesetPlan {
    repository: Repository!
    fileDiffs(first: Int): PreviewFileDiffConnection!
}

type PreviewFileDiffConnection {
    nodes: [PreviewFileDiff!]!
    totalCount: Int
    pageInfo: PageInfo!
    diffStat: DiffStat!
    rawDiff: String!
}

type ExternalChangeset {
    repository: Repository!
    diff: RepositoryComparison
	... others
}

type RepositoryComparison {
    baseRepository: Repository!
    headRepository: Repository!
    range: GitRevisionRange!
    commits(first: Int): GitCommitConnection!
    fileDiffs(first: Int): FileDiffConnection!
}

type FileDiffConnection {
    nodes: [FileDiff!]!
    totalCount: Int
    pageInfo: PageInfo!
    diffStat: DiffStat!
    rawDiff: String!
}

If we would reintroduce the RepositoryComparison on the preview level, the structure would be more aligned and it would help a lot creating more broadly usable components for both previews and actual campaigns/changesets :)

So I'm suggesting here that we reintroduce

type PreviewRepositoryComparison {
    baseRepository: Repository!
    fileDiffs(first: Int): PreviewFileDiffConnection!
}