Regression: 500 on diff download on merge request

When you try to download a MR as email patch or plain diff gitlab 7.12 throws a 500 error.
This is reproducable on gitlab.com.

edit:
Log entry:

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:86:in `format_patch'
  app/models/merge_request.rb:277:in `to_patch'
  app/controllers/projects/merge_requests_controller.rb:54:in `block (2 levels) in show'
  actionpack (4.1.11) lib/action_controller/metal/mime_responds.rb:258:in `respond_to'
  app/controllers/projects/merge_requests_controller.rb:50:in `show'
  actionpack (4.1.11) lib/action_controller/metal/implicit_render.rb:4:in `send_action'
  actionpack (4.1.11) lib/abstract_controller/base.rb:189:in `process_action'
  actionpack (4.1.11) lib/action_controller/metal/rendering.rb:10:in `process_action'
  actionpack (4.1.11) lib/abstract_controller/callbacks.rb:20:in `block in process_action'
  activesupport (4.1.11) lib/active_support/callbacks.rb:113:in `call'
  activesupport (4.1.11) lib/active_support/callbacks.rb:552:in `block (2 levels) in compile'
  activesupport (4.1.11) lib/active_support/callbacks.rb:502:in `call'
  activesupport (4.1.11) lib/active_support/callbacks.rb:86:in `run_callbacks'
  actionpack (4.1.11) lib/abstract_controller/callbacks.rb:19:in `process_action'
  actionpack (4.1.11) lib/action_controller/metal/rescue.rb:29:in `process_action'
  actionpack (4.1.11) lib/action_controller/metal/instrumentation.rb:32:in `block in process_action'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `instrument'
  actionpack (4.1.11) lib/action_controller/metal/instrumentation.rb:30:in `process_action'
  actionpack (4.1.11) lib/action_controller/metal/params_wrapper.rb:250:in `process_action'
  activerecord (4.1.11) lib/active_record/railties/controller_runtime.rb:18:in `process_action'
  actionpack (4.1.11) lib/abstract_controller/base.rb:136:in `process'
  actionview (4.1.11) lib/action_view/rendering.rb:30:in `process'
  rack-mini-profiler (0.9.0) lib/mini_profiler/profiling_methods.rb:108:in `block in profile_method'
  actionpack (4.1.11) lib/action_controller/metal.rb:196:in `dispatch'
  actionpack (4.1.11) lib/action_controller/metal/rack_delegation.rb:13:in `dispatch'
  actionpack (4.1.11) lib/action_controller/metal.rb:232:in `block in action'
  actionpack (4.1.11) lib/action_dispatch/routing/route_set.rb:82:in `dispatch'
  actionpack (4.1.11) lib/action_dispatch/routing/route_set.rb:50:in `call'
  actionpack (4.1.11) lib/action_dispatch/journey/router.rb:73:in `block in call'
  actionpack (4.1.11) lib/action_dispatch/journey/router.rb:59:in `call'
  actionpack (4.1.11) lib/action_dispatch/routing/route_set.rb:692:in `call'
  request_store (1.0.5) lib/request_store/middleware.rb:9:in `call'
  rack-cors (0.2.9) lib/rack/cors.rb:54:in `call'
  rack-attack (4.3.0) lib/rack/attack.rb:104:in `call'
  warden (1.2.3) lib/warden/manager.rb:35:in `block in call'
  warden (1.2.3) lib/warden/manager.rb:34:in `call'
  rack (1.5.5) lib/rack/etag.rb:23:in `call'
  rack (1.5.5) lib/rack/conditionalget.rb:25:in `call'
  rack (1.5.5) lib/rack/head.rb:11:in `call'
  actionpack (4.1.11) lib/action_dispatch/middleware/params_parser.rb:27:in `call'
  actionpack (4.1.11) lib/action_dispatch/middleware/flash.rb:254:in `call'
  rack (1.5.5) lib/rack/session/abstract/id.rb:225:in `context'
  rack (1.5.5) lib/rack/session/abstract/id.rb:220:in `call'
  actionpack (4.1.11) lib/action_dispatch/middleware/cookies.rb:562:in `call'
  activerecord (4.1.11) lib/active_record/query_cache.rb:36:in `call'
  activerecord (4.1.11) lib/active_record/connection_adapters/abstract/connection_pool.rb:621:in `call'
  actionpack (4.1.11) lib/action_dispatch/middleware/callbacks.rb:29:in `block in call'
  activesupport (4.1.11) lib/active_support/callbacks.rb:82:in `run_callbacks'
  actionpack (4.1.11) lib/action_dispatch/middleware/callbacks.rb:27:in `call'
  actionpack (4.1.11) lib/action_dispatch/middleware/reloader.rb:73:in `call'
  actionpack (4.1.11) lib/action_dispatch/middleware/remote_ip.rb:76:in `call'
  better_errors (1.0.1) lib/better_errors/middleware.rb:84:in `protected_app_call'
  better_errors (1.0.1) lib/better_errors/middleware.rb:79:in `better_errors_call'
  better_errors (1.0.1) lib/better_errors/middleware.rb:56:in `call'
  actionpack (4.1.11) lib/action_dispatch/middleware/debug_exceptions.rb:17:in `call'
  actionpack (4.1.11) lib/action_dispatch/middleware/show_exceptions.rb:30:in `call'
  railties (4.1.11) lib/rails/rack/logger.rb:38:in `call_app'
  railties (4.1.11) lib/rails/rack/logger.rb:20:in `block in call'
  activesupport (4.1.11) lib/active_support/tagged_logging.rb:68:in `block in tagged'
  activesupport (4.1.11) lib/active_support/tagged_logging.rb:26:in `tagged'
  activesupport (4.1.11) lib/active_support/tagged_logging.rb:68:in `tagged'
  railties (4.1.11) lib/rails/rack/logger.rb:20:in `call'
  quiet_assets (1.0.2) lib/quiet_assets.rb:18:in `call_with_quiet_assets'
  actionpack (4.1.11) lib/action_dispatch/middleware/request_id.rb:21:in `call'
  rack (1.5.5) lib/rack/methodoverride.rb:21:in `call'
  rack (1.5.5) lib/rack/runtime.rb:17:in `call'
  rack (1.5.5) lib/rack/lock.rb:17:in `call'
  actionpack (4.1.11) lib/action_dispatch/middleware/static.rb:84:in `call'
  lib/gitlab/middleware/static.rb:9:in `call'
  rack (1.5.5) lib/rack/sendfile.rb:112:in `call'
  rack-mini-profiler (0.9.0) lib/mini_profiler/profiler.rb:298:in `call'
  railties (4.1.11) lib/rails/engine.rb:514:in `call'
  railties (4.1.11) lib/rails/application.rb:144:in `call'
  railties (4.1.11) lib/rails/railtie.rb:194:in `method_missing'
  rack (1.5.5) lib/rack/builder.rb:138:in `call'
  rack (1.5.5) lib/rack/urlmap.rb:65:in `block in call'
  rack (1.5.5) lib/rack/urlmap.rb:50:in `call'
  unicorn (4.6.3) lib/unicorn/http_server.rb:552:in `process_client'
  unicorn (4.6.3) lib/unicorn/http_server.rb:632:in `worker_loop'
  unicorn (4.6.3) lib/unicorn/http_server.rb:500:in `spawn_missing_workers'
  unicorn (4.6.3) lib/unicorn/http_server.rb:142:in `start'
  unicorn (4.6.3) bin/unicorn_rails:209:in `<top (required)>'
   () home/hannes/.gem/ruby/2.1.5/bin/unicorn_rails:23:in `<main>'

Also note that I wasn't able to reproduce this as admin. But it occurs when I'm logged out.