copies: fix the changeset based algorithm regarding merge

In 99ebde4fec99, we changed the list of files stored into the `files` field.
This lead to the changeset centric copy algorithm to break in various merge
situation involving merge. Older information could reach the merge through
`p1`, and while information from `p2` was strictly fresher, it would get
overwritten anyway.

We update the situation with more details about which revision introduces rename
information. This help use making the right decision in case of merge.

We are now running a more comprehensive suite of test with include this kind of
situation. The behavior differ slightly from the filelog based in a couple of
instance. There is mostly two distinct cases:

1) there are conflicting rename information in a merge (different rename history
on each side). In this case the filelog based implementation arbitrarily pick a
side based on the file-revision-number. So it depends on a local factor. The
changeset centric algorithm will use a deterministic approach, by picking the
information coming from the first parent of the merge. This is stable across
different clone.

2) rename information related to file that exist in both source and destination.
The filelog based implementation do not even try to detect these, however the
changeset centric one get them for "free" (it is simpler to detect them than
not).

The new implementation focus on correctness. Performance improvement will come
later.

Differential Revision: https://phab.mercurial-scm.org/D8244
10 jobs for topic/default/test-copies in 29 minutes and 41 seconds (queued for 7 minutes and 3 seconds)
Status Job ID Name Coverage
  Test
passed checks-py2 #71679

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passed checks-py3 #71680

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passed rust-cargo-test-py2 #71681

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passed rust-cargo-test-py3 #71682

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passed test-py2 #71683

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passed test-py2-pure #71685

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passed test-py2-rust #71687

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passed test-py3 #71684

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passed test-py3-pure #71686

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passed test-py3-rust #71688

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