fix(ui2): incomplete :echon message in g< pager #37819

Problem:
`:echo 1 | echon 2<cr>g<` shows "2", but should be "12".

Solution:
Don't clear temp msg (g<) if we are appending.

runtime/lua/vim/_core/ui2/messages.lua

@@ -255,7 +255,9 @@ function M.show_msg(tar, content, replace_last, append, id)
       for _, chunk in ipairs(content) do
         msg = msg .. chunk[2]
       end
-      dupe = (not extid and msg == M.prev_msg and ui.cmd.srow == 0 and M.dupe + 1 or 0)
+      dupe = (
+        not extid and not append and msg == M.prev_msg and ui.cmd.srow == 0 and M.dupe + 1 or 0
+      )
     end
 
     cr = next(M[tar].ids) ~= nil and msg:sub(1, 1) == '\r'