Summary:
Ref PHI1749. Instead of opening files to the last unchanged line on either side of the change, open files to the "simple" line number of the selected block.

For inlines, this is the inline line number.

For blocks, this is the first new-file line number, or the first old-file line number if no new-file line number exists in the block.

This may not always be what the user is hoping for (we can't know what the state of their working copy is) but should produce more obvious behavior.

Test Plan:
  - In Diffusion, used "Open in Editor" with and without line selections. Saw same behavior as before.
  - Used "n" and "r" to leave an inline with the keyboard, saw same behavior as before.
  - Used "\" and "Open in Editor" menu item to open a file with:
    - Nothing selected or changeset selected (line: 1).
    - An inline selected (line: inline line).
    - A block selected (line: first line in block, per above).

Differential Revision: https://secure.phabricator.com/D21282