copies: split creation of the graph and actual checking again
The re-install the old split. It will be necessary to test that the upgrade
process produced a functionally identical result. It will be useful to detect
case where the metadata we look at identical, but some other items we did not
checked are missing.
(spoiler: we will find some bug)
Differential Revision: https://phab.mercurial-scm.org/D9195