Add structured diff support to Gitlab

[rohit-gohri]

Currently gitlab platform doesn't support diffs either through a FullDiff implementation or through structured diffs:

danger-js/source/platforms/gitlab/GitLabGit.ts

Lines 14 to 21 in 6e4371b

	// TODO: implement me when the API methods are in
	getFullDiff: async (): Promise<string> => {
	throw new Error("getFullDiff is not yet implemented")
	},
	// TODO: implement me when the API methods are in
	getStructuredDiffForFile: async (): Promise<GitStructuredDiff> => {
	throw new Error("getStructuredDiffForFile is not yet implemented")
	},

Gitlab exposes a full .diff file for merge requests: eg. https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/27117.diff
But that only works for public repos, can't access it through API (AFAIK) so not viable to use.

But it's API exposes individual file diffs in Compare API: https://docs.gitlab.com/ce/api/repositories.html#compare-branches-tags-or-commits

The format is a bit different from standard git diff, some things are parsed and removed from it (Open Issue: https://gitlab.com/gitlab-org/gitlab/issues/24913).

So added a function to parse this diff format and create a full diff from it similar to one in danger-ruby:
https://github.com/danger/danger/blob/aa490b4ca59b4b13acaea8f2b11372d06baeec03/lib/danger/request_sources/gitlab.rb#L95-L107

[rohit-gohri]

@jamime @notjosh @bigkraig If either of you could take a look that'll be great!

[orta]

I'm into the PR 👍 but I'm a tad worried about how much type safety we lose to make it work