Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

Rendering ten thousands of tree items consumes a lot of server
time and can cause timeouts in extreme cases. Realistically,
displaying more than 1,000 files is probably not useful so
truncate and show the user a notice instead. 'Find files' can be
used to locate specific files beyond the 1,000 limit.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Because the post-processing of the rendered README is dependent on the
context (i.e. the current user), do the post-processing when the
README is being displayed.

Currently trees are sorted in the fashion:
- folders
- files
- submodules
with each section sorted alphabetically

This changes to this system:
- folders and submodules (sorted together)
- files

Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/12785
Merge Request: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3514