fix(messages): add append parameter to history entries (#34467)

Problem:  The "append" parameter added in abb40ece is missing from
          history entries, resulting in different message formatting
          for "g<".
Solution: Add "append" field to message history entries.

Co-authored-by: phanium <91544758+phanen@users.noreply.github.com>

src/nvim/message.c

@@ -1053,6 +1053,11 @@ static void msg_hist_add_multihl(HlMessage msg, bool temp)
   entry->kind = msg_ext_kind;
   entry->prev = msg_hist_last;
   entry->next = NULL;
+  // NOTE: this does not encode if the message was actually appended to the
+  // previous entry in the message history. However append is currently only
+  // true for :echon, which is stored in the history as a temporary entry for
+  // "g<" where it is guaranteed to be after the entry it was appended to.
+  entry->append = msg_ext_append;
 
   if (msg_hist_first == NULL) {
     msg_hist_first = entry;
@@ -1206,6 +1211,7 @@ void ex_messages(exarg_T *eap)
         ADD(content, ARRAY_OBJ(content_entry));
       }
       ADD(entry, ARRAY_OBJ(content));
+      ADD(entry, BOOLEAN_OBJ(p->append));
       ADD(entries, ARRAY_OBJ(entry));
     }
     if (redirecting() || !ui_has(kUIMessages)) {

src/nvim/message_defs.h

@@ -18,4 +18,6 @@ typedef struct msg_hist {
   HlMessage msg;          ///< Highlighted message.
   const char *kind;       ///< Message kind (for msg_ext)
   bool temp;              ///< Temporary message since last command ("g<")
+  bool append;            ///< Message should be appended to previous entry, as opposed
+                          ///< to on a new line (|ui-messages|->msg_show->append).
 } MessageHistoryEntry;