Fix ignored

[Lightborne]

• Add test to sanity-check basic behavior of ignored() method
• Add test to verify GitCommandError is raised if git check-ignore returns any error code other than 1
• Modify ignred() method to raise GitCommandError if git check-ignore returns any error code other than 1

[Byron]

Great work, thanks so much! I love to finally have a test for that.

For a moment I thought this might be a breaking change, just to realize that the previous implementation was just a little too 'unconditional' and this PR implements the proper fix by raising an error when the error code does not indicate a lack of matches.

[Lightborne]

Thanks so much!