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 serve
$ hg clone http://localhost:$HGPORT/ copy
abort: * (glob)
[255]
$ test -d copy
[1]
$ "$PYTHON" "$TESTDIR/dumbhttp.py" -p $HGPORT --pid dumb.pid
$ cat dumb.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/foo copy2
abort: HTTP Error 404: * (glob)
[255]
$ killdaemons.py