tests/test-dirstate-backup.t

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

Set up

  $ hg init repo
  $ cd repo

Try to import an empty patch

  $ hg import --no-commit - <<EOF
  > EOF
  applying patch from stdin
  abort: stdin: no diffs found
  [255]

No dirstate backups are left behind

  $ ls .hg/dirstate* | sort
  .hg/dirstate