Improved large commit handling.
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.