Don't create an issue for commits that are already merged

[tesseralis]

Currently, chetsumi creates an issue for every item in the RSS feed, even those that are already merged in. For example: https://github.com/reactjs/ja.reactjs.org/issues?q=is%3Aissue+is%3Aclosed

All the [Doc] issues generated by che-tsumi are for commits that already existed when the repo was branched.

You can check that a commit already exists like I did here in my fork: https://github.com/tesseralis/che-tsumi/blob/master/lib/repository.js#L80

[re-fort]

Nice feature😍
I'm thinking abount separating lib folder from che-tsumi and creating che-tsumi core.
After I do that, feel free to make a PR when you add a new feature!

[tesseralis]

Will do! ^_^