Fix: strdisplaywidth("x", 2) returned 3 instead of 1.

src/eval.c

@@ -16820,7 +16820,7 @@ f_strdisplaywidth(argvars, rettv)
     if (argvars[1].v_type != VAR_UNKNOWN)
 	col = get_tv_number(&argvars[1]);
-    rettv->vval.v_number = (varnumber_T)(linetabsize_col(col, s));
+    rettv->vval.v_number = (varnumber_T)(linetabsize_col(col, s) - col);
 }
 /*