Add flag to promote only last git diff

[saschagrunert]

We can filter the manifest parse results based on which files or entries have changed in the PR and on the k8s.io main branch:

promo-tools/internal/legacy/dockerregistry/schema/manifest.go

Lines 251 to 321 in 6ecd71c

	// ParseThinManifestsFromDir parses all thin Manifest files within a directory.
	// We effectively have to create a map of manifests, keyed by the source
	// registry (there can only be 1 source registry).
	func ParseThinManifestsFromDir(
	dir string,
	) ([]Manifest, error) {
	mfests := make([]Manifest, 0)
	// Check that the thin manifests dir follows a regular, predefined format.
	// This is to ensure that there isn't any funny business going on around
	// paths.
	if err := ValidateThinManifestDirectoryStructure(dir); err != nil {
	return mfests, err
	}
	var parseAsManifest filepath.WalkFunc = func(
	path string,
	info os.FileInfo,
	err error,
	) error {
	if err != nil {
	// Prevent panic in case of incoming errors accessing this path.
	logrus.Errorf("failure accessing a path %q: %v\n", path, err)
	}
	// Skip directories (because they are not YAML files).
	if info.IsDir() {
	return nil
	}
	// First try to parse the path as a manifest file, which must be named
	// "promoter-manifest.yaml". This restriction is in place to limit the
	// scope of what is read in as a promoter manifest.
	if filepath.Base(path) != "promoter-manifest.yaml" {
	return nil
	}
	// If there are any files named "promoter-manifest.yaml", they must be
	// inside a subfolder within "manifests/<dir>" --- any other paths are
	// forbidden.
	shortened := strings.TrimPrefix(path, dir)
	shortenedList := strings.Split(shortened, "/")
	if len(shortenedList) != ThinManifestDepth {
	return fmt.Errorf("unexpected manifest path %q",
	path)
	}
	mfest, errParse := ParseThinManifestFromFile(path)
	if errParse != nil {
	logrus.Errorf("could not parse manifest file '%s'\n", path)
	return errParse
	}
	// Save successful parse result.
	mfests = append(mfests, mfest)
	return nil
	}
	// Only look at manifests starting with the "manifests" subfolder (no need
	// to walk any other toplevel subfolder).
	if err := filepath.Walk(filepath.Join(dir, "manifests"), parseAsManifest); err != nil {
	return mfests, err
	}
	if len(mfests) == 0 {
	return nil, fmt.Errorf("no manifests found in dir: %s", dir)
	}
	return mfests, nil
	}

The tricky part is to find the right diff for PRs as well as main. A new flag can be introduced to test the option later on.

Refers to #637