graftcopies: use _filter() for filtering out invalid copies `graftcopies()` (formerly called `duplicatecopies()`) checked that the copy destination existed in the working copy, but it didn't check that copy source existed in the parent of the working copy. In `test-graft.t` we can see that as warnings about not finding ancestors of the copied files, and also empty commits getting created. This patch uses the existing `_filter()` function for filtering out invalid copies. In addition to the aforementioned types, that also includes copies where source and destination is the same. Differential Revision: https://phab.mercurial-scm.org/D7859