transplant: added support for --stop flag
This adds fuctionality for `--stop` flag to `transplant`.
A new method `stop` is added to `transplanter` class
containing logic to abort transplant.
Tests are updated as shown.
Differential Revision: https://phab.mercurial-scm.org/D6695
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors