Arcyd: create a mock implementation of conduit

For faster and more comprehensive unit testing, start implementing a
mock version of conduit; this will allow us to more tightly control the
expectations which Arcyd must satisfy.

Move the unit tests over to use the mock version, the production code
will now use the real version; thus introducing the possibility of
missing errors.

Test Plan: $ ./precommit.sh
Arcyd testbed: reduce delay in run_arcyd_multi.sh

Reduced delay from 5 secs to 2 secs

Test Plan: testbed/arcyd/ $ ./run_arcyd_multi.sh
Arcyd: abstract Git usage as abdt_git

To make refactoring more tractable and to make mocking possible, extract
all interaction with Git into a new class and module.

Test Plan: $ ./precommit.sh
abdi_processrepo__t: extract phab/dev git use

So that we can later mock the Git interactions, extract them into a
separate class which lives in a separate module.

Test Plan: $ ./precommit.sh
Minor, re-order abdi_processrepo__t imports