Arcyd: fix exception when landing msg not ascii

Fix an exception that occurs if the commit message suggested by
the Phabricator server for a particular review by the
'get commit message' conduit API cannot be converted to ascii.

Introduce a new module to lossily convert to ascii from unicode and use
that to post-process the commit message from Phabricator.

Test Plan:
poked in test plan with 'horizontal ellipses', exercised with shell:
$ ./testbed/arcyd/test_shell.sh

$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
$ ./testbed/arcyd/loop_test.sh
Arcyd: test_shell - make accept.sh available

Add a new 'accept.sh' available in the 'test shell' environment so that
you can use 'poke.sh' to create new reviews and 'accept.sh' to accept
those reviews without going into the web interface.

Test Plan:
Poke and accept in
$ ./testbed/arcyd/test_shell.sh