Tweak to support really big merge requests

Related to:

https://gitlab.com/gitlab-org/gitlab-ce/issues/295#note_160691

Fix based on Matthieu Vachon suggestion.
It mean if something bad happens - satellite will be blocked for 600 seconds (10mins). Nobody will be able to edit files or create merge requests. Another problem is that web server unicorn timeout still `30` seconds. So changing only this value does not help. 

If satellite cant collect diff in 30 seconds then it is something huge there. What sense to create MR for such big changes if you wont be able to review it anyway?