Repositories - don't send emails or publish feed stories about commits from importing repositories

Summary: ...also truncate authorName to 255 so that we don't get database errors. Ref T6350.

Test Plan: see T6350 - mostly doing it live - but I did sanity check and commit something and it worked!

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: Korvin, epriestley

Maniphest Tasks: T6350

Differential Revision: https://secure.phabricator.com/D10734