Correct minor "jump to symbol" behavior in Differential
- 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- Fix an outdated reference to ".path", which provided context for symbol lookup.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.
- 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.
Differential Revision: https://secure.phabricator.com/D21641