[RFC] Remove 2-iteration loop in syn_combine_list() (rebase)

Rebased #692, reviewed it carefully again, improved the comments, and tested it as I described in https://github.com/neovim/neovim/pull/692#issuecomment-48123606

Preserved @harsh1kumar commit.
Since `list_op` is only checked for `CLUSTER_ADD`, `bool cluster cluster_add` would be more clear.