Introduce Compare model in the codebase.

app/controllers/projects/compare_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/models/commit.rb

app/models/compare.rb

app/models/merge_request.rb

app/services/compare_service.rb

app/services/merge_requests/build_service.rb

...rvices/merge_requests/merge_request_diff_cache_service.rb

app/views/projects/commit/show.html.haml

app/views/projects/compare/show.html.haml

app/views/projects/diffs/_diffs.html.haml

app/views/projects/merge_requests/_new_submit.html.haml

app/views/projects/merge_requests/show/_diffs.html.haml

app/workers/emails_on_push_worker.rb

lib/gitlab/diff/file_collection/base.rb

lib/gitlab/diff/file_collection/commit.rb

lib/gitlab/diff/file_collection/compare.rb

lib/gitlab/diff/file_collection/merge_request.rb

lib/gitlab/email/message/repository_push.rb

spec/lib/gitlab/email/message/repository_push_spec.rb