Add and use phlgitu_fixture.CentralisedWithWorkers

Introduce new helper classes in phlgitu_fixture to make it easy to
simulate a workflow where there is one central, canonical repository
with workers contributing and sharing code to and from it.

Update abdt_branch__t and abdt_git__t to use the new helpers.

Test Plan:
Exercised by abdt_branch__t and abdt_git__t
$ ./precommit.sh