Remember user's inline/tabular diff view preference in a cookie

As per #3071 (closed), some users (we have a local EE installation) prefer 2-column view in diff. In this MR I add an implementation for this feature, using a cookie.

👍 @dzaporozhets, what do you think?

@stanhu I think this is very useful for people who use split diff regularly. Can you check if implementation works smooth?

PTAL.

What do you think about using an if block here?

e.g.:

if view = params[:view] || cookies[:diff_view]
  cookies.permanent[:diff_view] = params[:view] = view
end

To me, as a C++ user, assignment in an if statement is one of the most obscure ways to write code. I do not know Ruby and its idiom well, but if I read your question literally, as in what I think about this code style, then I think this if statement would be extremely, utterly confusing to code readers. If, however, your point is that I must write this way according to GitLab or general Ruby code style, I will grudgingly rewrite it; a pointer to said code style would be very welcome in this case though!

Or maybe this would be more readable?

view = params[:view] || cookies[:diff_view]
if view
  cookies.permanent[:diff_view] = view
  params[:view] = view
end

Fair enough! Actually, the Ruby style-guide we're using tells "Favor modifier if/unless usage when you have a single-line body" (https://github.com/bbatsov/ruby-style-guide#if-as-a-modifier) so let's keep the original code as it is since the body is a single-line (even if there are actually two assignments in it).

Regarding the style guides, they are listed in our CONTRIBUTING.md document. :)

@kkm Could you please add an acceptance test for this feature?

mentioned in merge request !2723 (merged)

Status changed to closed

@kkm Thanks for your work, I've submitted a rebased merge request with a test at !2723 (merged).

mentioned in commit 9fdd605f