patch 8.2.4731: the changelist index is not remembered per buffer

Problem:    The changelist index is not remembered per buffer.
Solution:   Keep the changelist index per window and buffer. (closes #10135,
            closes #2173)
diff --git a/src/testdir/test_changelist.vim b/src/testdir/test_changelist.vim
index 872b87a..f429968 100644
--- a/src/testdir/test_changelist.vim
+++ b/src/testdir/test_changelist.vim
@@ -1,6 +1,28 @@
 " Tests for the changelist functionality
 
 " Tests for the getchangelist() function
+func Test_changelist_index()
+  edit Xfile1.txt
+  exe "normal iabc\<C-G>u\ndef\<C-G>u\nghi"
+  call assert_equal(3, getchangelist('%')[1])
+  " Move one step back in the changelist.
+  normal 2g;
+
+  hide edit Xfile2.txt
+  exe "normal iabcd\<C-G>u\ndefg\<C-G>u\nghij"
+  call assert_equal(3, getchangelist('%')[1])
+  " Move to the beginning of the changelist.
+  normal 99g;
+
+  " Check the changelist indices.
+  call assert_equal(0, getchangelist('%')[1])
+  call assert_equal(1, getchangelist('#')[1])
+
+  bwipe!
+  call delete('Xfile1.txt')
+  call delete('Xfile2.txt')
+endfunc
+
 func Test_getchangelist()
   bwipe!
   enew
@@ -11,6 +33,7 @@
   call writefile(['line1', 'line2', 'line3'], 'Xfile2.txt')
 
   edit Xfile1.txt
+  let buf_1 = bufnr()
   exe "normal 1Goline\<C-G>u1.1"
   exe "normal 3Goline\<C-G>u2.1"
   exe "normal 5Goline\<C-G>u3.1"
@@ -22,6 +45,7 @@
 	      \ getchangelist('%'))
 
   hide edit Xfile2.txt
+  let buf_2 = bufnr()
   exe "normal 1GOline\<C-G>u1.0"
   exe "normal 2Goline\<C-G>u2.0"
   call assert_equal([[
@@ -33,10 +57,12 @@
   call assert_equal([[
 	      \ {'lnum' : 2, 'col' : 4, 'coladd' : 0},
 	      \ {'lnum' : 4, 'col' : 4, 'coladd' : 0},
-	      \ {'lnum' : 6, 'col' : 4, 'coladd' : 0}], 3], getchangelist(2))
+	      \ {'lnum' : 6, 'col' : 4, 'coladd' : 0}], 2],
+	      \ getchangelist(buf_1))
   call assert_equal([[
 	      \ {'lnum' : 1, 'col' : 6, 'coladd' : 0},
-	      \ {'lnum' : 3, 'col' : 6, 'coladd' : 0}], 2], getchangelist(3))
+	      \ {'lnum' : 3, 'col' : 6, 'coladd' : 0}], 2],
+	      \ getchangelist(buf_2))
 
   bwipe!
   call delete('Xfile1.txt')