clipboard: remove start_batch_changes() in redir_write()

start_batch_changes() doesn't avoid invoking the clipboard
once-per-line, because the loop is actually in ex_echo(), which calls
redir_write() for each message. But we've already entered
start_batch_changes() by then, so that was never the problem.

    redir_write at /home/vagrant/old.neovim/build/../src/nvim/message.c:2523
    msg_puts_attr_len at /home/vagrant/old.neovim/build/../src/nvim/message.c:1600
    msg_outtrans_len_attr at /home/vagrant/old.neovim/build/../src/nvim/message.c:1221
    ex_echo at /home/vagrant/old.neovim/build/../src/nvim/eval.c:19433
    do_one_cmd at /home/vagrant/old.neovim/build/../src/nvim/ex_docmd.c:2242

Trying to defer _explicit_ clipboard updates is difficult.
    :redir @+ | silent echo system('cat foo') | redir END
is essentially equivalent to:
    for l in readfile('foo')
        let @+ .= l
    endfor
We cannot make judgements about when to ignore a script's bad decisions.
start_batch_changes() only works around the case of clipboard=unnamed,
i.e. _implicit_ clipboard updates (`:g/foo/d`).  Not explicit
assignment.

src/nvim/message.c

@@ -2519,7 +2519,6 @@ static void redir_write(const char *const str, const ptrdiff_t maxlen)
   if (redirecting()) {
     /* If the string doesn't start with CR or NL, go to msg_col */
     if (*s != '\n' && *s != '\r') {
-      start_batch_changes();
       while (cur_col < msg_col) {
         if (capture_ga) {
           ga_concat_len(capture_ga, " ", 1);
@@ -2536,7 +2535,6 @@ static void redir_write(const char *const str, const ptrdiff_t maxlen)
         }
         cur_col++;
       }
-      end_batch_changes();
     }
 
     size_t len = maxlen == -1 ? STRLEN(s) : (size_t)maxlen;

src/nvim/ops.c

@@ -5558,7 +5558,7 @@ static yankreg_T *adjust_clipboard_name(int *name, bool quiet, bool writing)
       msg((char_u *)MSG_NO_CLIP);
       clipboard_didwarn_unnamed = true;
     }
-    // ... else, be silent (avoid a flood of messages).
+    // ... else, be silent (don't flood during :while, :redir, etc.).
     goto end;
   }
 
@@ -5567,10 +5567,12 @@ static yankreg_T *adjust_clipboard_name(int *name, bool quiet, bool writing)
     goto end;
   } else {  // unnamed register: "implicit" clipboard
     if (writing && clipboard_delay_update) {
+      // For "set" (copy), defer the clipboard call.
       clipboard_needs_update = true;
       goto end;
     } else if (!writing && clipboard_needs_update) {
-      goto end;  // use the internal value
+      // For "get" (paste), use the internal value.
+      goto end;
     }
 
     if (cb_flags & CB_UNNAMEDPLUS) {
@@ -5772,6 +5774,7 @@ void end_batch_changes(void)
   }
   clipboard_delay_update = false;
   if (clipboard_needs_update) {
+    // unnamed ("implicit" clipboard)
     set_clipboard(NUL, y_previous);
     clipboard_needs_update = false;
   }