What does hg copy do?

[Christoph Mathys]

We recently did a 'hg copy' of a directory in our repo. We thought it
does something like 'cp -a' and 'hg add' and maybe flag somehow that
this file has been copied from another file inside the repo (so 'hg
annotate' shows the original committer). But it now seems that 'hg
copy' does more or different stuff than that. I couldn't really find
much on how exactly copy works. So:

- What exactly does hg copy do and what special treatment does this
cause in the future?
- If it turns out to do 'the wrong thing(tm)' for our case, how do I
unflag the file as beeing a copy of another file?

Christoph