(sheldon): fix sheldon errors with submodules

What does this MR do?

• Exclude submodules in files to process by sheldon
• Fix sheldon check on a single commit, even a merge commit using git show

How to test it?

To test submodule correction

Run sheldon with a range of commit on a repository with submodule:

> sheldon XXXX YYYY

To test single commit check

Run sheldon on a commit on a git repository (even merge commit)

> sheldon my_commit

Data

No required data

Additional tests to run

Run sheldon unit test:

> cd ./hooks
> python -m unittest discover

Associated issue

Closes #20 (closed)