Handle copying directory into itself

[nabijaczleweli]

#95 (comment)

Ref: #61
Ref: #95

[thecoshman]

Are we needing to make a call about how to handle this? I think I'm ok with having this simple not a thing you can do, but don't really see much reason why it can't be supported.

[nabijaczleweli]

No, I would just need to copy an algorithm from rsync instead of stealing one from elsewhere