Fix routing bug that prevents from seing file named `diff`

Before this commit, it was impossible to see a blob whose filename ends in `diff` because GET `path/to/diff` routed to `blob#diff`.

Currently live on GitLab: https://gitlab.com/cirosantilli/test0/blob/master/diff

You will get redirected to; http://localhost:3000/gitlab-org/gitlab-test/tree/master/, where you can see the the file named `diff` does exist!
Prefer single-quoted strings when you don't need string interpolation or special symbols.