Directly navigating to a submodule leads to "empty file"
### Summary
Attempting to view a submodule in the web interface results shows an 'empty file'. There's no link to this in the UI, but a user might reasonably navigate to this page:
http://gitlab.example.com/group/project/blob/master/my-submodule
### What is the current *bug* behavior?
The file displays as "Empty file"
### What is the expected *correct* behavior?
"Submodule at [SHA]"
(I'm using gitlab master at, arbitrarily, gitlab-ce@b2e1d32e7a74b22b23e8ab052bd29e15bb8a4f1e)
issue