Collapsed diffs lines/size don't accumulate to overflow diffs.

CHANGELOG

Gemfile

Gemfile.lock

app/controllers/concerns/diff_for_path.rb

app/helpers/diff_helper.rb

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

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

lib/gitlab/diff/file.rb

spec/features/expand_collapse_diffs_spec.rb

spec/helpers/diff_helper_spec.rb

spec/lib/gitlab/diff/file_spec.rb

spec/support/test_env.rb