vim-patch:8.2.3820: "vrc" does not replace composing characters

Problem:    "vrc" does not replace composing characters, while "rc" does.
Solution:   Check the byte length including composing characters.
            (closes vim/vim#9351)
8ee6028de3

vim-patch:8.2.3823: test for visual replace is in wrong function

Problem:    Test for visual replace is in wrong function.
Solution:   Move it to another function.
6ecf58b0d7

src/nvim/testdir/test_visual.vim

@@ -636,6 +636,11 @@ func Test_characterwise_visual_mode()
   normal v$rx
   call assert_equal(['x'], getline(1, '$'))
 
+  " replace a character with composing characters
+  call setline(1, "xã̳x")
+  normal gg0lvrb
+  call assert_equal("xbx", getline(1))
+
   bwipe!
 endfunc