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;
   }

test/functional/clipboard/clipboard_provider_spec.lua

@@ -93,17 +93,16 @@ describe('clipboard', function()
     local screen = Screen.new(72, 8)
     screen:attach()
     command("let g:clipboard = 'bogus'")
-    feed_command('redir @+> | :call system("cat CONTRIBUTING.md") | redir END')
+    feed_command('redir @+> | :silent echo system("cat CONTRIBUTING.md") | redir END')
-    -- it is made empty
     screen:expect([[
+      ^                                                                        |
+      ~                                                                       |
+      ~                                                                       |
+      ~                                                                       |
       ~                                                                       |
       ~                                                                       |
       ~                                                                       |
-      Error detected while processing function provider#clipboard#Executable: |
-      line    5:                                                              |
-      clipboard: invalid g:clipboard                                          |
       clipboard: No provider. Try ":CheckHealth" or ":h clipboard".           |
-      Press ENTER or type command to continue^                                 |
     ]], nil, {{bold = true, foreground = Screen.colors.Blue}})
   end)
 end)
@@ -118,7 +117,23 @@ describe('clipboard', function()
     feed_command('call getreg("*")') -- force load of provider
   end)
 
-   it('has independent "* and unnamed registers per default', function()
+  it('`:redir @+>` invokes clipboard once-per-message', function()
+    eq(0, eval("g:clip_called_set"))
+    feed_command('redir @+> | :silent echo system("cat CONTRIBUTING.md") | redir END')
+    -- Assuming CONTRIBUTING.md has >100 lines.
+    assert(eval("g:clip_called_set") > 100)
+  end)
+
+  it('`:redir @">` does not invoke clipboard', function()
+    -- :redir to a non-clipboard register, with `:set clipboard=unnamed`.
+    -- Does _not_ propagate to the clipboard (complies with Vim behavior).
+    command("set clipboard=unnamedplus")
+    eq(0, eval("g:clip_called_set"))
+    feed_command('redir @"> | :silent echo system("cat CONTRIBUTING.md") | redir END')
+    eq(0, eval("g:clip_called_set"))
+  end)
+
+  it('has independent "* and unnamed registers per default', function()
     insert("some words")
     feed('^"*dwdw"*P')
     expect('some ')
@@ -325,7 +340,7 @@ describe('clipboard', function()
 
   end)
 
-  describe('with clipboard=unnamedplus', function()
+  describe('clipboard=unnamedplus', function()
     before_each(function()
       feed_command('set clipboard=unnamedplus')
     end)
@@ -369,6 +384,7 @@ describe('clipboard', function()
         really unnamed
         the plus]])
     end)
+
     it('is updated on global changes', function()
       insert([[
 	text

test/functional/fixtures/autoload/provider/clipboard.vim

@@ -5,7 +5,13 @@ let s:methods = {}
 let g:cliplossy = 0
 let g:cliperror = 0
 
+" Count how many times the clipboard was invoked.
+let g:clip_called_get = 0
+let g:clip_called_set = 0
+
 function! s:methods.get(reg)
+  let g:clip_called_get += 1
+
   if g:cliperror
     return 0
   end
@@ -19,6 +25,8 @@ function! s:methods.get(reg)
 endfunction
 
 function! s:methods.set(lines, regtype, reg)
+  let g:clip_called_set += 1
+
   if a:reg == '"'
     call s:methods.set(a:lines,a:regtype,'+')
     call s:methods.set(a:lines,a:regtype,'*')