copies: detect case when a merge decision overwrite previous data
We now detect and record when a merge case required special logic (eg: thing
that append during the merge, ambiguity leading to picking p1 data, etc) and we
explicitly mark the result as superseding the previous data.
This fixes the family of test we previously added.
Differential Revision: https://phab.mercurial-scm.org/D9613
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors