Move Arcyd 'create empty revision' to new conduit

Replace direct dependencies on phlsys_conduit with the
abdt_conduit abstraction.

Test Plan: $ ./precommit.sh
Move Arcyd 'get_commit_message' to new conduit

Replace direct dependencies on phlsys_conduit with the
abdt_conduit abstraction.

Test Plan: $ ./precommit.sh
Arcyd: remove redundant act_as_user_context

Remove a redundant act_as_user_context, this allows us to unsplit some
lines due to the new indentation level.

Test Plan: $ ./precommit.sh
Move last Arcyd as_user context to new conduit

Replace direct dependencies on phlsys_conduit with the
abdt_conduit abstraction.

Test Plan: $ ./precommit.sh