Add ignore white space option in merge request diff

CHANGELOG

app/controllers/projects/compare_controller.rb

app/models/merge_request.rb

app/models/merge_request_diff.rb

app/services/compare_service.rb

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

doc/workflow/merge_requests.md

doc/workflow/merge_requests/commit_compare.png

doc/workflow/merge_requests/merge_request_diff.png

.../merge_requests/merge_request_diff_without_whitespace.png

lib/gitlab/compare_result.rb

spec/controllers/projects/compare_controller_spec.rb

spec/controllers/projects/merge_requests_controller_spec.rb