Merge branch 'fix-diff-patch-public-mr' into 'master'

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
* Closes #1854 (dup)
* Closes #1858 (dup)

See merge request !872

2 files changed, 7 insertions, 3 deletions

diff --git a/lib/gitlab/backend/shell_env.rb b/lib/gitlab/backend/shell_env.rb
index 044afb27f3f..17ec029eed4 100644
--- a/lib/gitlab/backend/shell_env.rb
+++ b/lib/gitlab/backend/shell_env.rb
@@ -6,7 +6,9 @@ module Gitlab
 
     def set_env(user)
       # Set GL_ID env variable
-      ENV['GL_ID'] = "user-#{user.id}"
+      if user
+        ENV['GL_ID'] = "user-#{user.id}"
+      end
     end
 
     def reset_env
diff --git a/lib/gitlab/satellite/action.rb b/lib/gitlab/satellite/action.rb
index 4890ccf21e6..489070f1a3f 100644
--- a/lib/gitlab/satellite/action.rb
+++ b/lib/gitlab/satellite/action.rb
@@ -39,8 +39,10 @@ module Gitlab
       def prepare_satellite!(repo)
         project.satellite.clear_and_update!
 
-        repo.config['user.name'] = user.name
-        repo.config['user.email'] = user.email
+        if user
+          repo.config['user.name'] = user.name
+          repo.config['user.email'] = user.email
+        end
       end
 
       def default_options(options = {})