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)

runtime/doc/channel.txt

@@ -1,4 +1,4 @@
-*channel.txt*      For Vim version 8.2.  Last change: 2020 Jul 10
+*channel.txt*      For Vim version 8.2.  Last change: 2020 Sep 03
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -608,6 +608,10 @@ ch_logfile({fname} [, {mode}])					*ch_logfile()*
 		after every message, on Unix you can use "tail -f" to see what
 		is going on in real time.
 
+		To enable the log very early, to see what is received from a
+		terminal during startup, use |--cmd|: >
+			vim --cmd "call ch_logfile('logfile', 'w')"
+<
 		This function is not available in the |sandbox|.
 		NOTE: the channel communication is stored in the file, be
 		aware that this may contain confidential and privacy sensitive
@@ -1256,7 +1260,9 @@ After setting 'buftype' to "prompt" Vim does not automatically start Insert
 mode, use `:startinsert` if you want to enter Insert mode, so that the user
 can start typing a line.
 
-The text of the prompt can be set with the |prompt_setprompt()| function.
+The text of the prompt can be set with the |prompt_setprompt()| function. If
+no prompt is set with |prompt_setprompt()|, "% " is used. You can get the
+effective prompt text for a buffer, with |prompt_getprompt()|.
 
 The user can go to Normal mode and navigate through the buffer.  This can be
 useful to see older output or copy text.

runtime/doc/eval.txt

@@ -2684,6 +2684,7 @@ popup_show({id})		none	unhide popup window {id}
 pow({x}, {y})			Float	{x} to the power of {y}
 prevnonblank({lnum})		Number	line nr of non-blank line <= {lnum}
 printf({fmt}, {expr1}...)	String	format text
+prompt_getprompt({buf})		String	get prompt text
 prompt_setcallback({buf}, {expr}) none	set prompt callback function
 prompt_setinterrupt({buf}, {text}) none	set prompt interrupt function
 prompt_setprompt({buf}, {text}) none	set prompt text
@@ -7835,6 +7836,17 @@ printf({fmt}, {expr1} ...)				*printf()*
 		arguments an error is given.  Up to 18 arguments can be used.
 
 
+prompt_getprompt({buf})					*prompt_getprompt()*
+		Returns the effective prompt text for buffer {buf}. {buf} can
+		be a buffer name or number. |prompt-buffer|.
+
+		If the buffer doesn't exist or isn't a prompt buffer, an empty
+		string is returned.
+
+		Can also be used as a |method|: >
+			GetBuffer()->prompt_getprompt()
+
+
 prompt_setcallback({buf}, {expr})			*prompt_setcallback()*
 		Set prompt callback for buffer {buf} to {expr}.  When {expr}
 		is an empty string the callback is removed.  This has only
@@ -7890,7 +7902,7 @@ prompt_setprompt({buf}, {text})				*prompt_setprompt()*
 		Can also be used as a |method|: >
 			GetBuffer()->prompt_setprompt('command: ')
 
-prop_ functions are documented here: |text-prop-functions|.
+prop_ functions are documented here: |text-prop-functions|
 
 pum_getpos()						*pum_getpos()*
 		If the popup menu (see |ins-completion-menu|) is not visible,

runtime/doc/usr_41.txt

@@ -1118,6 +1118,7 @@ Tags:						*tag-functions*
 	settagstack()		modify the tag stack of a window
 
 Prompt Buffer:					*promptbuffer-functions*
+	prompt_getprompt()	get the effective prompt text for a buffer
 	prompt_setcallback()	set prompt callback for a buffer
 	prompt_setinterrupt()	set interrupt callback for a buffer
 	prompt_setprompt()	set the prompt text for a buffer