Mention that users can edit the hooks
Remove unused require statement
Use Tempfile instead of `sed -i`

The syntax for `sed -i` is incompatible between GNU sed and
BSD sed. By Tempfile from the Ruby standard library we can
avoid using the `-i` option of sed.
Merge branch 'comments_in_default_hooks' of /home/git/repositories/gitlab/gitlab-shell
Merge branch 'avoid_sed_i' of /home/git/repositories/gitlab/gitlab-shell