rebase: derive node from target rev (issue3802)

dest.rev() is the same as target when a new rebase is run, but dest
isn't set when rebase --continue is run. Bug introduced in 2a1fac3650a5,
which fixed issue3685.

--HG--
branch : stable
extra : rebase_source : 481acdc2d576ac695a483101f620b3fb05924f08