Don't overwrite identical files

Check if newly generated (compressed) files are the same as the target
file. "Same" means if they only have different timestamps but the
content matches. This has to be done by uncompressing both files and
recursively diff their contents as zdiff & co fail to do that properly.
Merge pull request #1 from saschpe/master-avoid-overwriting-identical-files

Don't overwrite identical files