Move code to different places, add comments here and there, no actual changes

internal/backer/xz.go

@@ -0,0 +1,17 @@
+package backer
+
+import (
+	"context"
+	"io"
+
+	"github.com/ulikunitz/xz"
+)
+
+// CreateTarXzStream takes a list of files and returns a reader that reads from an xz-compressed tar archive.
+func CreateTarXzStream(ctx context.Context, filepaths []string) io.ReadCloser {
+	return createArchiveStream(ctx, filepaths, func(w io.Writer) (io.WriteCloser, error) {
+		return xz.NewWriter(w)
+	})
+}
+
+/* vim: setlocal ft=go noet ai ts=4 sw=4 sts=4: */