Only add a comment if one does not already exist

[edhgoose]

Hey,

We have a few cases where having no id is sort of ok (temporary branches, hacking around, sharing an idea quickly), and at the moment every time we push to that branch, we get a new comment.

This stops the comment from being added if it already exists.

Thanks!

(We did the same with Pivotal Lint over here too: https://github.com/ClearTax/pivotal-lint/pull/28, which we're no longer using)