perf: Disable clipboard in do_cmdline().

For any script--not just `:global` commands--there is no reason to
update the system clipboard until the script is finished, so disable it
during do_cmdline().

Before this change, 'clipboard=unnamedplus' causes scripted editing to
be extremely slow (e.g. `:normal` in a while-loop).

Closes #3534

src/nvim/ex_cmds.c

@@ -4053,9 +4053,7 @@ void ex_global(exarg_T *eap)
       smsg(_("Pattern not found: %s"), pat);
     }
   } else {
-    start_batch_changes();
     global_exe(cmd);
-    end_batch_changes();
   }
   ml_clearmarked();        /* clear rest of the marks */
   vim_regfree(regmatch.regprog);

src/nvim/ex_cmds2.c

@@ -1983,8 +1983,6 @@ void ex_listdo(exarg_T *eap)
     save_ei = au_event_disable(",Syntax");
   }
 
-  start_batch_changes();
-
   if (eap->cmdidx == CMD_windo
       || eap->cmdidx == CMD_tabdo
       || P_HID(curbuf)
@@ -2181,7 +2179,6 @@ void ex_listdo(exarg_T *eap)
     apply_autocmds(EVENT_SYNTAX, curbuf->b_p_syn,
                    curbuf->b_fname, true, curbuf);
   }
-  end_batch_changes();
 }
 
 /// Add files[count] to the arglist of the current window after arg "after".

src/nvim/ex_docmd.c

@@ -346,6 +346,7 @@ int do_cmdline(char_u *cmdline, LineGetter fgetline,
     return FAIL;
   }
   ++call_depth;
+  start_batch_changes();
 
   cstack.cs_idx = -1;
   cstack.cs_looplevel = 0;
@@ -953,6 +954,7 @@ int do_cmdline(char_u *cmdline, LineGetter fgetline,
   did_endif = FALSE;    /* in case do_cmdline used recursively */
 
   --call_depth;
+  end_batch_changes();
   return retval;
 }
 
@@ -9513,8 +9515,6 @@ static void ex_folddo(exarg_T *eap)
 {
   linenr_T lnum;
 
-  start_batch_changes();
-
   /* First set the marks for all lines closed/open. */
   for (lnum = eap->line1; lnum <= eap->line2; ++lnum)
     if (hasFolding(lnum, NULL, NULL) == (eap->cmdidx == CMD_folddoclosed))
@@ -9523,8 +9523,6 @@ static void ex_folddo(exarg_T *eap)
   /* Execute the command on the marked lines. */
   global_exe(eap->arg);
   ml_clearmarked();        /* clear rest of the marks */
-
-  end_batch_changes();
 }
 
 static void ex_terminal(exarg_T *eap)