Improved large commit handling.

Previously, only number of changed files mattered. Now, the 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.