Enable sparseness only conditionally

We can either preallocate storage for a file or sparsify it. This
detects a pack file as sparse if it contains an all zero block or
consists of only one block. As the file sparsification is just an
approximation, hide it behind a `--sparse` parameter.

internal/restorer/sparsewrite.go

@@ -8,6 +8,10 @@ import "bytes"
 // WriteAt writes p to f.File at offset. It tries to do a sparse write
 // and updates f.size.
 func (f *partialFile) WriteAt(p []byte, offset int64) (n int, err error) {
+	if !f.sparse {
+		return f.File.WriteAt(p, offset)
+	}
+
 	n = len(p)
 	end := offset + int64(n)