bufhl: use extmark column adjustment for bufhl

NB: this is not the final implementation. Bufhl should be made a
part of the extmark tree, so that "start" adjustment just works
automatically. But "stop" will still need some ad-hoc trickery,
until extended marks natively support ranges (hopefully sooner than
forever).

src/nvim/edit.c

@@ -5599,9 +5599,10 @@ insertchar (
     do_digraph(-1);                     /* clear digraphs */
     do_digraph(buf[i-1]);               /* may be the start of a digraph */
     buf[i] = NUL;
+    colnr_T col_start = curwin->w_cursor.col;
     ins_str(buf);
     extmark_col_adjust(curbuf, curwin->w_cursor.lnum,
-                       (colnr_T)(curwin->w_cursor.col + 1), 0,
+                       (colnr_T)(col_start + 1), 0,
                        (long)STRLEN(buf), kExtmarkUndo);
     if (flags & INSCHAR_CTRLV) {
       redo_literal(*buf);