test-copies: introduce merge chains test for the P/Q merges
This is similar to the chaining test we have for the A/E merges.
The current result of this tests is wrong, as for the other test of the same
familly. This will be fixed by a later changesets.
Differential Revision: https://phab.mercurial-scm.org/D10059
(binary:application/octet-stream)