Arcyd: more detail on 'no history' error

Update commenter to leave a more helpful message when a review has been
created from a branch which seems to introduce no additional commits.

Make suggestions as to next actions for the (guessed) author.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

view results of
$ ./testbed/arcyd/test_shell.sh
    $ git push HEAD:r/master/no_history