Apply soft diff limits to MergeRequest#new page

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'fix-new-mr-502' into 'master'

Fix 502 error on new merge request page

When diff is huge we try to render it that cause 502 error often.
After this patch we apply soft limits to diffs. So if diff is huge - we doing show it

Fixes #1323