code: function cloneGitRepo(host, ref) {
      var isBranch = false;
  
      ref = ref || 'master';
      if (!validateSha(ref)) isBranch = true;
  
      return Q()
  
      // Create temporary folder to store git repos
      .then(function() {
          if (GIT_TMP) return;
          return fs.tmp.dir()
              .then(function(_tmp) {
                  GIT_TMP = _tmp;
              });
      })
  
      // Return or clone the git repo
      .then(function() {
          // Unique ID for repo/ref combinaison
          var repoId = crc.crc32(host+'#'+ref).toString(16);
  
          // Absolute path to the folder
          var repoPath = path.resolve(GIT_TMP, repoId);
  
          return fs.exists(repoPath)
              .then(function(doExists) {
                  if (doExists) return;
  
                  // Clone repo
                  return exec('git clone '+host+' '+repoPath)
                  .then(function() {
                      return exec('git checkout '+ref, { cwd: repoPath });
                  });
              })
              .thenResolve(repoPath);
      });
  }
docs: Clone a git repo from a specific ref