Fix an issue with symbol lookup identifying path names in Diffusion

Summary:
Depends on D18939. Ref T13047. Symbol lookup can be activated from a diff (in Differential or Diffusion) or from the static view of a file at a particular commit.

In the latter case, we need to figure out the path a little differently. The character and line number approaches still work as written.

Test Plan:

• Command-clicked symbols in the Diffusion browse view with blame on and off; saw path, line and char populate properly.
• Command-clicked symbols in Differential diff view to check I didn't break anything.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13047

Differential Revision: https://secure.phabricator.com/D18940