copies: add test that makes both the merging csets dirty and fails
This patch is a part of series which is about the case when both the merging
csets are not descendant of merge base. The existing code assumes if c1 is
dirty there shouldn't be any partial copies from c2 i.e both2['incomplete']
and same for c2, if c2 is dirty both1['incomplete'] should be empty,
but this is not the right assumption.
Now as we know we can have both c1 and c2 dirty at the same time, it
is possible that c1 is dirty and both2['incomplete'] has some value.
Or if c2 is dirty and both1['incomplete'] has some value.
Added test shows that because of this assumption it could fail.
Differential Revision: https://phab.mercurial-scm.org/D5962