R1:ad7755d9a993

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…