Add terraform_module_shallow_clone rule

[chernetskyi]

Closes #256

[bendrucker]

Nice! There are some tricky details to commit detection but overall this looks good. Left some suggestions around tightening up the documentation.

[bendrucker]

Thanks for keeping after this! There's still some work to be done to harden the commit vs branch/tag detection. Even with the rule disabled by default, emitting invalid advice for unpunctuated branch names is a costly false positive.

The key to more sophisticated behavior will be having a very comprehensive suite of test cases to verify against.