manifest: now that there's only one legacy.py, exclude it
After the last patch, this file is only included because of package_dir in
setup.py (i.e. just once), so we can ignore it with just one clause. But
package_dir starts with "./", so we need to add it to the pattern.
extend-diff-ignore = "^(contrib/|.gitlab-ci.yml|.hg-format-source|MANIFEST|Makefile|docs/tutorial/.netlify|hgext3rd/evolve/hack/|tests/.testtimes)"