Make InlineCommentQueries more robust/consistent

Summary:
Ref T13513. Improve consistency and robustness of the "InlineComment" queries.

The only real change here is that these queries now implicitly add a clause for selecting inlines ("pathID IS NULL" or "changesetID IS NULL").

Test Plan: Browed, created, edited, and submitted inlines.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21227