Fix broken file browsing with relative submodule in personal projects

Git submodules with relative links work fine for projects in groups but not in personal projects. This patch fixes this issue.

To reproduce the problem, go here:

https://gitlab.com/andrewwu.tw/submodule-test/tree/master

Closes https://github.com/gitlabhq/gitlabhq/issues/7554

See merge request !565