Problem: crash when passing invalid buffer to undotree()
Solution: Use get_buf_arg() instead of tv_get_buf_from_arg().
closes: vim/vim#12862closes: vim/vim#12830ab9f2ecfd4
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: vim/vim#4001closes: vim/vim#122925fee111149
Co-authored-by: Devin J. Pohly <djpohly@gmail.com>
