Previously, only number of changed files mattered. Now, number of lines to render in the diff are also taken into account.
A hard limit is set, above which diffs are not rendered and users are not allowed to override that. This prevents high server
resource usage with huge commits.
Related to #1745, #2259
In addition, handle large commits for MergeRequests and Compare controllers.
Also fixes a bug where diffs are loaded twice, if user goes directly to merge_requests/:id/diffs URL.
