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/edit.c b/src/edit.c
index dc0b450..bc74f44 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -1682,16 +1682,26 @@
 
 #if defined(FEAT_JOB_CHANNEL) || defined(PROTO)
 /*
+ * Return the effective prompt for the specified buffer.
+ */
+    char_u *
+buf_prompt_text(buf_T* buf)
+{
+    if (buf->b_prompt_text == NULL)
+	return (char_u *)"% ";
+    return buf->b_prompt_text;
+}
+
+/*
  * Return the effective prompt for the current buffer.
  */
     char_u *
 prompt_text(void)
 {
-    if (curbuf->b_prompt_text == NULL)
-	return (char_u *)"% ";
-    return curbuf->b_prompt_text;
+    return buf_prompt_text(curbuf);
 }
 
+
 /*
  * Prepare for prompt mode: Make sure the last line has the prompt text.
  * Move the cursor to this line.