R1:db9191f9a8d5

Correct minor "jump to symbol" behavior in Differential

Summary:
Ref T13644. Ref T13638.

- Double-encode the symbol that is used as a path component, similar to Diffusion.
- Fix an outdated reference to ".path", which provided context for symbol lookup.
- Prevent command-clicking headers from looking up the path as a symbol.

Test Plan:
- Command-clicked headers, no longer got a symbol.
- Command-clicked stuff with "/", saw it double-encoded and decoded properly.
- Command-clicked normal…