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

src/infrastructure/markup/PhabricatorMarkupEngine.php

@@ -586,7 +586,7 @@ final class PhabricatorMarkupEngine {
     //  - Hopefully don't return too much text. We don't explicitly limit
     //    this right now.
 
-    $blocks = preg_split("/\n *\n\s*/", trim($corpus));
+    $blocks = preg_split("/\n *\n\s*/", $corpus);
 
     $best = null;
     foreach ($blocks as $block) {