[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.
Comment to clarify this:
```c
      // Add only unique elements and elements that are not removed
      // (list_op == CLUSTER_SUBTRACT) from the result list.
```