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):

{F399841}

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