gistfile1.js

var git = require('gulp-git');
var inject = require('inject-string');
var q = require('q');

var getHash = function() {
  var deferred = q.defer();
  $.git.revParse({args:'--short HEAD'}, function (err, hash) {
    deferred.resolve(hash);
  });
  return deferred.promise;
};

gulp.task('html', function () {
  getHash().then(function(data) {
    return data;
  }).then(function(hash) {
    return gulp.src('app/index.html')
      .pipe(inject.append(hash))
      .pipe(gulp.dest('dist'))  
  }
});