Raw submodule displays as empty blob in web UI

Summary

The web UI displays raw blobs from a repo given a commit and path, but if you pass the path to a submodule, you get back an empty blob.

Steps to reproduce

click on (or curl) https://gitlab.com/abliss/bugdemo/raw/c65fb343/submodule

Example Project

https://gitlab.com/abliss/bugdemo

What is the current bug behavior?

An empty blob is returned. See e.g. these headers:

HTTP/1.1 200 OK
Content-Length: 0

Note that this provides no information, and worse, is indistinguishable from actually having an empty file at that path.

What is the expected correct behavior?

It's debatable what should be returned. In order of my personal preference, I would suggest:

1. return a length-40 text/plain page with the sha of the submodule, i.e. the same output as git rev-parse $COMMIT:$PATH_TO_SUBMODULE. In this case, 0c0e98372b42d70ecb1a3e7e3b002d40763321ac. This is most likely to give the user the information they're looking for, but could conflict with a text file at that path.
2. Return a text/plain page with the output of git ls-tree $COMMIT $PATH_TO_SUBMODULE, in this case 160000 commit 0c0e98372b42d70ecb1a3e7e3b002d40763321ac submodule. This is perhaps a bit less likely to conflict with a file at that path, and is more self-evident as being a submodule.
3. Do one of the above, but set the Http Status to something like 214 Transformation Applied and/or use an HTTP Header to distinguish between a raw blob and the submodule output.
4. Fail the request, perhaps with 415 Unsupported Media Type or 501 Not Implemented or something.
5. Fail the request with 404 Not found, which seems like an inappropriate status code, but at least would be consistent with what happens when you pass a path to a tree. E.g. https://gitlab.com/gitlab-org/gitlab-ce/raw/c134a72c/tmp

Output of checks

This bug happens on GitLab.com