diff: normalize path before considering the difference

Because of the way our rootfs tarball was generated, we would have the entry like system/./android and system/android which was treated as a different entry. However, MR 9 [1] removes the latter path, resulting in diff that removes system/android despite the earlier entry stil exists.

Thus, normalize the paths before comparing the difference between 2 tarballs. Although, care has to be taken to use unnormalized path when accessing the file in both tarballs.

A test is added to catch this issue, and to also test the unnormalized paths in general.

[1] !9 (merged)

Fixes: #7 (closed)