.arcconfig

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

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