(issue1561)
Before this change, newancestor was used only once as a replacement
for ancestor.ancestor, but merge.update calls ancestor.ancestor
several times, so it ends up with the "wrong" ancestor (the real
ancestor, but we want the parent of the rebased changeset for all but
the first rebased changeset).
Added a new test case for this: test-rebase-newancestor.
Also, in one scenario in test-rebase-collapse, there was a spurious
conflict caused by the same issue, so that test case was fixed by
removing the now unneeded conflict resolution and the output was
adapted accordingly.
