vim-patch:9.1.0113: duplicate code when cleaning undo stack

Problem:  duplicate code when cleaning undo stack
Solution: refactor undo cleanup into a single public function

related: vim/vim#13928

9071ed8107

Co-authored-by: Christian Brabandt <cb@256bit.org>

src/nvim/fileio.c

@@ -3171,8 +3171,7 @@ void buf_reload(buf_T *buf, int orig_mode, bool reload_options)
       // Mark the buffer as unmodified and free undo info.
       unchanged(buf, true, true);
       if ((flags & READ_KEEP_UNDO) == 0) {
-        u_blockfree(buf);
-        u_clearall(buf);
+        u_clearallandblockfree(buf);
       } else {
         // Mark all undo states as changed.
         u_unchanged(curbuf);