code: public void setBranches(String branches) {
                  if (!GitTaskUtils.isNullOrBlankString(branches)) {
                          this.branchNames = branches.split(",");
                  }
                  else {
                          throw new GitBuildException("Cannot delete unspecified branches.");
                  }
          }
docs: Sets the branches to delete (comma-separated list)
  
  @param branches comma-separated list of branches to delete