Handle symlinks

[ItsNickBarry]

lint-staged tests against symlinks in a few contexts:

• .git/ directory
• lint-staged config
• staged files

exec-staged needs defined behavior for these situations.

[ItsNickBarry]

lint-staged supports situations where the .git/ directory is symlinked to another directory within the same git repository, which may not be in .gitignore. This seems pointless, and exec-staged should probably just exit with an error.

[ItsNickBarry]

Tests added for symlinked config and staged files.

What happens if symlinks are chained together?