copies: explicitly filter out existing file in graftcopies
The `graftcopies` function does something very strange (maybe even wrong), it
calls `_filter` with a pair of changeset that does not match the one used to
compute the copies informations.
We are about to do some rework of `_filter` to make it closer to its documented
intent and fix a couple of bug. This means some of the logic that only make
sense for graft need to go somewhere else. We add the extra filtering with
proper documentation to `graftcopies`.
Differential Revision: https://phab.mercurial-scm.org/D9802