patch 9.0.1686: undotree() only works for the current buffer

Problem:    undotree() only works for the current buffer
Solution:   Add an optional "buffer number" parameter to undotree().  If
            omitted, use the current buffer for backwards compatibility.

closes: #4001
closes: #12292

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Devin J. Pohly <djpohly@gmail.com>

runtime/doc/builtin.txt

@@ -707,7 +707,7 @@ trunc({expr})			Float	truncate Float {expr}
 type({expr})			Number	type of value {expr}
 typename({expr})		String	representation of the type of {expr}
 undofile({name})		String	undo file name for {name}
-undotree()			List	undo file tree
+undotree([{buf}])		List	undo file tree for buffer {buf}
 uniq({list} [, {func} [, {dict}]])
 				List	remove adjacent duplicates from a list
 utf16idx({string}, {idx} [, {countcc} [, {charidx}]])
@@ -10073,9 +10073,10 @@ undofile({name})					*undofile()*
 		Can also be used as a |method|: >
 			GetFilename()->undofile()
 
-undotree()						*undotree()*
-		Return the current state of the undo tree in a dictionary with
-		the following items:
+undotree([{buf}])					*undotree()*
+		Return the current state of the undo tree for the current
+		buffer, or for a specific buffer if {buf} is given.  The
+		result is a dictionary with the following items:
 		  "seq_last"	The highest undo sequence number used.
 		  "seq_cur"	The sequence number of the current position in
 				the undo tree.  This differs from "seq_last"

runtime/doc/todo.txt

@@ -5535,7 +5535,6 @@ Undo:
 -   Undo history wrong when ":next file" re-uses a buffer. (#5426) ex_next()
     should pass flag to do_argfile(), then to do_ecmd().  Is there a test for
     this?
--   Add buffer argument to undotree(). (#4001)
 -   Undo problem: "g-" doesn't go back, gets stuck. (Björn Linse, 2016 Jul 18)
 -   Undo message is not always properly displayed.  Patch by Ken Takata, 2013
     oct 3.  Doesn't work properly according to Yukihiro Nakadaira.

runtime/doc/usr_41.txt

@@ -1373,7 +1373,7 @@ Various:					*various-functions*
 	libcallnr()		idem, returning a number
 
 	undofile()		get the name of the undo file
-	undotree()		return the state of the undo tree
+	undotree()		return the state of the undo tree for a buffer
 
 	shiftwidth()		effective value of 'shiftwidth'