copy: add option to unmark file as copied
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 `hg
copy [--forget]` a `--at-rev` argument for marking/unmarking copies
after commit (usually with `--at-rev .`).
Differential Revision: https://phab.mercurial-scm.org/D8029
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