Snitch does not work with submodules

[rexim]

Steps to reproduce

• git clone --recursive https://github.com/tsoding/grub-gamepad repo↗
• cd grub-gamepad
• snitch report

Observed

Snitch fails with error: read grub: is a directory

Expected

I'm not sure what exactly is expected here. I see two options:

1. Snitch ignores any submodules
2. Snitch recursively goes into the submodules

The problem with the 2nd approach is that when you go into the submodule you need to reidentify the repo link. 1st is easier to implement than 2nd, but 2nd sounds more preferable.

In the scope of this issue we can simply implement 1st and file 2nd as a separate issue.