Fix "Revspec not found" errors when viewing diffs in a forked project with submodules

## What does this MR do?

This MR fixes an error that occurs when viewing diffs in a forked project with submodules.

### Are there points in the code the reviewer needs to double check?

Testing this code was tricky. The only way this problem shows up is if the origin project does NOT have the submodule update commit. The introduction of gitlab-test-fork serves that purpose: it contains a submodule update not present in gitlab-test.

### Why was this MR needed?

A user would receive a 500 error when trying to view a merge request with a submodule update. #1413 has details on how to reproduce this issue.

### What are the relevant issue numbers / [Feature requests](http://feedback.gitlab.com/)?

#1413

See merge request !512