Show a notice for diffs that are empty/too large

This builds upon https://gitlab.com/gitlab-org/gitlab_git/merge_requests/72 but will fail until said MR has been merged.

Fixes gitlab-org/gitlab-ce#13912, gitlab-org/gitlab-ce#13644, gitlab-org/gitlab-ce#13557, gitlab-org/gitlab-ce#13267, gitlab-org/gitlab-ce#3906

See merge request !3185