title: git error "Please tell me who you are." and Heroku
body: <p>I was working in <code>branch master</code>, and committing to Git repository. Everything worked fine. I connected new app to this repository on Heroku. </p>
  
  <p>I was committing to both Heroku and Git. Everything worked fine again (except I cannot run db:migrate on Heroku but that is another question...). After my last commit I run <code>git status</code> and received: <code>On branch master. Your branch is up-to-date with 'origin/master'.</code> </p>
  
  <p>Today I made some changes in my code, but suddenly I cannot commit - received error "Please tell me who you are." The only difference from previous commits is that had a migration, if it is of any importance. 
  When I run <code>git config --global --get user.email</code>command I get an empty line in return.</p>
  
  <p>Why have I suddenly lost connection to git? </p>