[11.1] Include rich_text in diff cache keys

See merge request gitlab/gitlabhq!2482

Conflicts used to take a `Repository` and pass that to
`Gitlab::Highlight.highlight`, which would call `#gitattribute` on the
repository. Now they use a `Gitlab::Git::Repository`, which didn't have that
method defined - but defining it on `Gitlab::Git::Repository` does make it
available on `Repository` through `method_missing`, so we can do that and both
cases will work.

Rename classes to (hopefully) clearer names while we're doing that.

This prepares the codebase for a Gitaly migration. See
https://gitlab.com/gitlab-org/gitaly/issues/553

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

Remove superfluous lib: true, type: redis, service: true, models: true, services: true, no_db: true, api: true

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

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.

This is more efficient for large files than performing a regex match on
every single line.

- Add match line header to expected result for `File#sections`.
- Lowercase CSS colours.
- Remove unused `diff_refs` keyword argument.
- Rename `parent` -> `parent_file`, to be more explicit.
- Skip an iteration when highlighting.