patch 8.2.5023: substitute overwrites allocated buffer

Problem:    Substitute overwrites allocated buffer.
Solution:   Disallow undo when in a substitute command.

src/testdir/test_substitute.vim

@@ -1013,6 +1013,28 @@ func Test_sub_change_window()
   delfunc Repl
 endfunc
 
+" This was undoign a change in between computing the length and using it.
+func Do_Test_sub_undo_change()
+  new
+  norm o0000000000000000000000000000000000000000000000000000
+  silent! s/\%')/\=Repl()
+  bwipe!
+endfunc
+
+func Test_sub_undo_change()
+  func Repl()
+    silent! norm g-
+  endfunc
+  call Do_Test_sub_undo_change()
+
+  func! Repl()
+    silent earlier
+  endfunc
+  call Do_Test_sub_undo_change()
+
+  delfunc Repl
+endfunc
+
 " Test for the 2-letter and 3-letter :substitute commands
 func Test_substitute_short_cmd()
   new