Sync command re-downloads existing files

Hey everyone!
`s4cmd` looks really cool, I love how fast the `s4cmd du` command scales with huge buckets.

Well done!

I did notice that `s4cmd sync s3://... /local/directory` actually re-downloads files that are already present in the local destination directory -- any way to prevent that?

Thanks,
Elad

Start at the s4cmd sync command and reproduce the download behavior with an existing local destination file. Trace how sync compares remote objects with local files; done means existing files are skipped when they do not need downloading.

Written by the indexing model from the issue text.