Fix an off-by-one error affecting mail rendering of inlines on the final line of a file

Summary: Depends on D19837. Ref T13216. See PHI985. There's an off-by-one error here between how inline comments store "length" and how context rendering treats "length". We need to add 1 to the length, but currently do it a little too early. Do it slightly later so that inlines on the final line of a file render properly.

Test Plan: Left an inline on the final line of a new file, saw it render properly in HTML mail.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19838