Fix downloading of patches on public merge requests when user logged out

What does this MR do?

This MR makes it possible to download a diff patch on a public merge request when a user is logged out.

Why was this MR needed?

An Error 500 would result when a user attempted to click on the "Email Patches" or "Plain Diff" button:

NoMethodError - undefined method `id' for nil:NilClass:
  lib/gitlab/backend/shell_env.rb:9:in `set_env'
  lib/gitlab/satellite/action.rb:20:in `in_locked_and_timed_satellite'
  lib/gitlab/satellite/merge_action.rb:49:in `diff_in_satellite'
  app/models/merge_request.rb:219:in `to_diff'
  app/controllers/projects/merge_requests_controller.rb:42:in `block (2 levels) in show'

What are the relevant issue numbers?

• Closes #1225 (closed)
• Closes #1854 (closed) (dup)
• Closes #1858 (closed) (dup)

Reassigned to @dzaporozhets

/cc: @haynes

@stanhu You are awesome!

@stanhu can you make ci green?

@haynes Haha.

@dzaporozhets Yeah, hit the retry button. Not sure why the Rspec tests were cancelled.

@jacobvosmaer any chance this makes it into the patch release as well?
It's not a regression because this has been broken for a long time, but it does fix a 500 error.

Status changed to merged

mentioned in commit 541f7675

@stanhu

Is this a 7.12 regression?

@haynes I like seeing bugs fixed too but I do not want to turn the 7-12-stable branch into a zoo.

@jacobvosmaer I understand that. It is not a 7.12 regression. This has been broken for at least 3 months.
I guess it'll just be in 7.13 then. That's completly fine with me.
I just thought I would ask because it fixes a 500 error.

OK 7.13 then. Thanks for asking @haynes.