patch 8.2.1588: cannot read back the prompt of a prompt buffer

Problem:    Cannot read back the prompt of a prompt buffer.
Solution:   Add prompt_getprompt(). (Ben Jackson, closes #6851)
diff --git a/src/channel.c b/src/channel.c
index 1a899e8..5bf561e 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -6368,6 +6368,29 @@
     set_callback(&buf->b_prompt_interrupt, &callback);
 }
 
+
+/*
+ * "prompt_getprompt({buffer})" function
+ */
+    void
+f_prompt_getprompt(typval_T *argvars, typval_T *rettv)
+{
+    buf_T	*buf;
+
+    // return an empty string by default, e.g. it's not a prompt buffer
+    rettv->v_type = VAR_STRING;
+    rettv->vval.v_string = NULL;
+
+    buf = tv_get_buf_from_arg(&argvars[0]);
+    if (buf == NULL)
+	return;
+
+    if (!bt_prompt(buf))
+	return;
+
+    rettv->vval.v_string = vim_strsave(buf_prompt_text(buf));
+}
+
 /*
  * "prompt_setprompt({buffer}, {text})" function
  */