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
abdt_git: internalize clone and remote name

To reduce dependencies and repetition, internalize the remote name and
git clone in abdt_git.Clone.

Fixup all the clients of abdt_git.Clone accordingly.

Test Plan: $ ./precommit.sh