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>

src/testdir/test_undo.vim

@@ -93,6 +93,53 @@ func FillBuffer()
   endfor
 endfunc
 
+func Test_undotree_bufnr()
+  new
+  let buf1 = bufnr()
+
+  normal! Aabc
+  set ul=100
+
+  " Save undo tree without bufnr as ground truth for buffer 1
+  let d1 = undotree()
+
+  new
+  let buf2 = bufnr()
+
+  normal! Adef
+  set ul=100
+
+  normal! Aghi
+  set ul=100
+
+  " Save undo tree without bufnr as ground truth for buffer 2
+  let d2 = undotree()
+
+  " Check undotree() with bufnr argument
+  let d = undotree(buf1)
+  call assert_equal(d1, d)
+  call assert_notequal(d2, d)
+
+  let d = undotree(buf2)
+  call assert_notequal(d1, d)
+  call assert_equal(d2, d)
+
+  " Switch buffers and check again
+  wincmd p
+
+  let d = undotree(buf1)
+  call assert_equal(d1, d)
+
+  let d = undotree(buf2)
+  call assert_notequal(d1, d)
+  call assert_equal(d2, d)
+
+  " Drop created windows
+  set ul&
+  new
+  only!
+endfunc
+
 func Test_global_local_undolevels()
   new one
   set undolevels=5