rebase: fix bug that prevented dry-run rebases from printing failures
As far as I can tell it should be fine to unconditionally skip
_prepareabortorcontinue if we're in the process of raising an Abort
here.
Differential Revision: https://phab.mercurial-scm.org/D6226
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors