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