Event.Smart lacks method to access commit_id

[cvrebert]

See the commit_id attribute documented on https://developer.github.com/v3/issues/events/

[dmarkov]

@cvrebert I will find a developer for the task soon...

[bibonix]

I am planning to work on this issue.

[bibonix]

Submitted #1855 with Event.Smart#commitId() returning Optional<String>. All CI checks are green.