When mapping phantom comments across changes, correct an off-by-one issue

Summary:
Ref T13617. When an inline comment is added inside a block of added lines, it currently ends up off-by-one when porting forward.

This is a disagreement between the mapping engine and the display engine about what "offset" means. Choose the simpler of the two interpretations.

Test Plan:

• Created a revision with the diff in T13617.
• Added an inline in the middle of the added block.
• Updated the revision with the same diff.
  • Before: inline incorrectly moves up by one line.
  • After: inline maps correctly.

Maniphest Tasks: T13617

Differential Revision: https://secure.phabricator.com/D21572