Run script when creating gitlab commit

I'm using gitlab, I want to get the current commit text on commit in my workflow script. 
My code:

exports.rule = entity.Issue.onChange({
  title: 'vcs-change',
  action: function(ctx) {
     // Run only commit gitlab 
  }
});

I want this code to run when the user makes a commit and get the current commit text
I thought this code is run when the user makes a commit
exports.rule =  entities.VcsChange = (function(){ 
  // Run when commit even 
});

but not working