Allow the fulltext index to select only transactions with comments

Summary:
Ref T12997. Although we can't query by transaction type (since we can't easily enumerate all possible types which may have comments -- inline types may also have comments), we can just check if there's a comment row or not.

This reduces the amount of garbage we need to load to rebuild indexes for unusual objects with hundreds and hundreds of mentions.

Test Plan:

• Used batch editor to mention a task 700 times.
• Indexed it before and after this change, saw index time drop from 1600ms → 160ms.
• Made some new comments on it, verified that they still indexed/queried properly.
• Browsed around, made normal transactions, made inline comments.
• Added a unique word to an inline comment, indexed revision, searched for word, found revision.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12997

Differential Revision: https://secure.phabricator.com/D18660