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

copies: replace duplicatecopies() by function that takes contexts The callers mostly have context objects, so let's avoid looking up the same context objects inside `duplicatecopies()`. I also renamed the function to `graftcopies()` since I think that better matches its purpose. I did it in the same commit so it's easier for extensions to switch between the functions. Differential Revision: https://phab.mercurial-scm.org/D7858

graft: extract repo[None] to a variable I plan to allow the caller pass in an overlayworkingctx, so this prepares for that. Differential Revision: https://phab.mercurial-scm.org/D7857