Don't trim remarkup corpora before summarizing

Summary:
Currently, lists like this:

- a
- b
- c

...get trimmed before summarization and end up looking like this after summarization:

- a
  - b
  - c

This produces the summary artifacts (first item at wrong indent level):

Instead, don't trim. This produces better summaries.

Test Plan: Saw a better summary of a list.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12794