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/proto/channel.pro b/src/proto/channel.pro
index 56a0167..a402177 100644
--- a/src/proto/channel.pro
+++ b/src/proto/channel.pro
@@ -56,6 +56,7 @@
 int job_stop(job_T *job, typval_T *argvars, char *type);
 void invoke_prompt_callback(void);
 int invoke_prompt_interrupt(void);
+void f_prompt_getprompt(typval_T *argvars, typval_T *rettv);
 void f_prompt_setcallback(typval_T *argvars, typval_T *rettv);
 void f_prompt_setinterrupt(typval_T *argvars, typval_T *rettv);
 void f_prompt_setprompt(typval_T *argvars, typval_T *rettv);
diff --git a/src/proto/edit.pro b/src/proto/edit.pro
index 6efe537..e2ec8dc 100644
--- a/src/proto/edit.pro
+++ b/src/proto/edit.pro
@@ -4,6 +4,7 @@
 void ins_redraw(int ready);
 int decodeModifyOtherKeys(int c);
 void edit_putchar(int c, int highlight);
+char_u *buf_prompt_text(buf_T* buf);
 char_u *prompt_text(void);
 int prompt_curpos_editable(void);
 void edit_unputchar(void);