UpdateMergeRequestsWorker attempts to retrieve blobs from wrong project

https://sentry.gitlab.net/gitlab/gitlabcom/issues/537883/

Slack conversation: https://gitlab.slack.com/archives/C8HG8D9MY/p1540320539000100

GRPC::NotFound: 5:GetRepoPath: not a git repository '/var/opt/gitlab/git-data/repositories/@hashed/4f/45/4f453f87a598bd4144b602d60ee995e0546b4304c4de4ff8b3e4ef58f4510ad7.git'
  grpc/generic/active_call.rb:31:in `check_status'
    fail GRPC::BadStatus.new_status_exception(
  grpc/generic/active_call.rb:180:in `attach_status_results_and_complete_call'
    recv_status_batch_result.check_status
  grpc/generic/active_call.rb:169:in `receive_and_check_status'
    attach_status_results_and_complete_call(batch_result)
  grpc/generic/active_call.rb:328:in `block in each_remote_read_then_finish'
    receive_and_check_status
  grpc/generic/active_call.rb:325:in `loop'
    loop do
...
(107 additional frame(s) were not displayed)

5:GetRepoPath: not a git repository '/var/opt/gitlab/git-data/repositories/@hashed/4f/45/4f453f87a598bd4144b602d60ee995e0546b4304c4de4ff8b3e4ef58f4510ad7.git'