500 error trying to get submodule from blob API
I'm trying to get a submodule commit via the [Raw file content](https://docs.gitlab.com/ce/api/repositories.html#raw-file-content) API
Assuming that `lib/libfoo` is a submodule (mode `160000`), this request gives me a 500 error:
```
curl --header "PRIVATE-token: secretsecret" https://gitlab.example.com/api/v3/projects/123/repository/blobs/master?filepath=lib/libfoo
```
In `/var/log/gitlab/gitlab-workhorse/current` I'm seeing the following error (timestamps removed for clarity):
```
SendBlob: sending "deadbeef12345678" for "/api/v3/projects/123/repository/blobs/master"
fatal: git cat-file: could not get object info
error: GET "/api/v3/projects/123/repository/blobs/master?filepath=lib/libfoo": SendBlob: get blob size: exit status 128
"GET /api/v3/projects/123/repository/blobs/master?filepath=lib/libfoo" 500 22 "" "curl/7.38.0" 0.025730
```
issue