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• Fix an outdated reference to ".path", which provided context for symbol lookup.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 symbols, saw "path" populate correctly.

Maniphest Tasks: T13644, T13638

Differential Revision: https://secure.phabricator.com/D21641