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