To unmark a file as copied, the user currently has to do this:
hg forget <dest>
hg add <dest>
The new command simplifies that to:
hg copy --forget <dest>
That's not a very big improvement, but I'm planning to also teach both
`hg copy` and `hg uncopy` a `-r` argument for marking/unmarking copies
after commit (usually with `-r .`).