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Move abdt_conduit.createEmptyRevision to phl

Make space for a new Abd-specific Conduit class by moving
abdt_conduit.createEmptyRevision to phl_differential, where it arguably
should have gone in the first place.

Remove the 'author' parameter of createEmptyRevision, it should be the
caller's responsibility or we need another wrapper function to help.

Test Plan: $ ./precommit.sh
Use new Arcyd conduit, step towards testability

Take the first step towards better testability by creating a new
Arcyd-specific conduit wrapper implementation, which we'll be able to
test independantly and replace with a mock version during tests.

Test Plan:
$ ./precommit.sh
testbed/arcyd/ $ ./run_arcyd_multi.sh
Move abdcmt_commenter to use abdt_conduit

Replace direct dependencies on phlsys_conduit with the
abdt_conduit abstraction.

Test Plan: $ ./precommit.sh
Move Arcyd revision create, update to new conduit

Replace direct dependencies on phlsys_conduit with the
abdt_conduit abstraction.

Test Plan: $ ./precommit.sh