refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

src/nvim/eval.c

@@ -8162,7 +8162,7 @@ repeat:
   while (src[*usedlen] == ':' && src[*usedlen + 1] == 'h') {
     valid |= VALID_HEAD;
     *usedlen += 2;
-    s = (char *)get_past_head((char_u *)(*fnamep));
+    s = get_past_head(*fnamep);
     while (tail > s && after_pathsep(s, tail)) {
       MB_PTR_BACK(*fnamep, tail);
     }