startup: go to buffer 2 if stdin is empty

If stdin is not a TTY we read it into buffer 1, as text. But if the
stdin pipe is empty, Nvim was most likely invoked for some other reason.
DWIM: select buffer 2 (if it exists). Example:

    echo file1 | xargs nvim

closes #8560
closes #8561
ref https://github.com/equalsraf/neovim-qt/issues/417

src/nvim/main.c

@@ -1410,6 +1410,12 @@ static void read_stdin(void)
   int save_msg_didany = msg_didany;
   set_buflisted(true);
   (void)open_buffer(true, NULL, 0);  // create memfile and read file
+  if (BUFEMPTY() && curbuf->b_next != NULL) {
+    // stdin was empty, go to buffer 2 (e.g. "echo file1 | xargs nvim"). #8561
+    msg_silent++;
+    do_buffer(DOBUF_GOTO, DOBUF_FIRST, FORWARD, curbuf->b_next->handle, 0);
+    msg_silent--;
+  }
   no_wait_return = false;
   msg_didany = save_msg_didany;
   TIME_MSG("reading stdin");