Merge branch 'fix-submodules-with-relative-links' into 'master'

Fix broken file browsing with a submodule that has a relative link

## What does this MR do?

This MR fixes an error that occurs when browsing a submodule with a relative link.

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

I re-wrote the function because I was confused by how the first one was supposed to work. Please review if it's clearer.

### Why was this MR needed?

A 500 Error would occur when using the file browser on a repo with a submodule. Here's how to reproduce the bug:

1. Start a new project in GitLab.
2. Clone git://git.gniibe.org/gnuk/gnuk.git/ locally.
3. Push repo to new project.
4. Click on "Files" in the project.

The .gitmodules file:
```
[submodule "chopstx"]
        path = chopstx
        url = ../../chopstx/chopstx.git
```
### What are the relevant issue numbers / [Feature requests](http://feedback.gitlab.com/)?

1. #775
2. #1385
3. https://github.com/gitlabhq/gitlabhq/issues/8153
4. https://github.com/gitlabhq/gitlabhq/issues/8881
5. https://github.com/gitlabhq/gitlabhq/issues/7554

See merge request !508

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