evolve: fix stabilization of orphan merge changesets in case of conflicts
As demonstrated by tests added in previous patch, if one of the parent of a
merge changeset has a new version and merge changeset gets, running `hg evolve`
to stabilize that if results in conflicts will lead to lose of other parent of
that merge changeset when we resolve conflicts and do `hg evolve --continue`.
This patch fixes the behavior, by checking whether we are continuing evolving a
merge changeset and if yes, setting the dirstate parents according to that.
-------------------------
Evolve Commands Reference
-------------------------
.. highlight:: none
amend
-----
.. hghelp:: amend
evolve
------
.. hghelp:: evolve
fold
----
.. hghelp:: fold
metaedit
--------
.. hghelp:: metaedit
next
----
.. hghelp:: next
obslog
------
.. hghelp:: obslog
pdiff
-----
.. hghelp:: pdiff
previous
--------
.. hghelp:: previous
prune
-----
.. hghelp:: prune
pstatus
-------
.. hghelp:: pstatus
split
-----
.. hghelp:: split
touch
-----
.. hghelp:: touch
uncommit
--------
.. hghelp:: uncommit