fix(messages): single msg_show event for :global command #37728

Problem:  "first" parameter added in 912388f5 for grouping printed lines
          in a single event is ineffective for the :global command.
Solution: Only set the message kind for the first printed line during a
          :global command.

src/nvim/ex_cmds.c

@@ -1676,6 +1676,8 @@ void print_line_no_prefix(linenr_T lnum, bool use_number, bool list)
   msg_prt_line(ml_get(lnum), list);
 }
 
+static bool global_need_msg_kind = false;  // Start new message only once during :global.
+
 /// Print a text line.  Also in silent mode ("ex -s").
 void print_line(linenr_T lnum, bool use_number, bool list, bool first)
 {
@@ -1688,9 +1690,10 @@ void print_line(linenr_T lnum, bool use_number, bool list, bool first)
 
   silent_mode = false;
   info_message = true;  // use stdout, not stderr
-  if (first) {
+  if ((!global_busy || global_need_msg_kind) && first) {
     msg_start();
     msg_ext_set_kind("list_cmd");
+    global_need_msg_kind = false;
   } else if (!save_silent) {
     msg_putchar('\n');  // don't want trailing newline with regular messaging
   }
@@ -4764,6 +4767,7 @@ void global_exe(char *cmd)
 
   sub_nsubs = 0;
   sub_nlines = 0;
+  global_need_msg_kind = true;
   global_need_beginline = false;
   global_busy = 1;
   old_lcount = curbuf->b_ml.ml_line_count;

test/functional/ui/messages_spec.lua

@@ -468,6 +468,20 @@ describe('ui/ext_messages', function()
       messages = { { content = { { 'line 1\nline ' } }, kind = 'list_cmd' } },
     })
 
+    -- single message for :global command #37726
+    feed(':g/line<CR>')
+    screen:expect({
+      grid = [[
+        line 1                   |
+        ^line                     |
+        {1:~                        }|*3
+      ]],
+      messages = {
+        { content = { { '\n' } }, kind = '' },
+        { content = { { 'line 1\nline ' } }, kind = 'list_cmd' },
+      },
+    })
+
     command('command Foo Bar')
     feed(':command<CR>')
     screen:expect({