R1:7cbbe2ccf793

When users browse to a submodule path in Diffusion explicitly, don't fatal

Summary: Ref T13030. See PHI254. This behavior could be cleaner than I've made it, but it fixes the "this is totally broken" issue, replacing a fatal/exception with an informative (just not terribly useful) page.

Test Plan:
- Added a submodule to a repository.
- In Diffusion, clicked some other file next to the submodule, then edited the URI to the submodule path instead.
- Before patch: fatal.
- After patch…