templates/tox: dedicates the job to Diff testing

means the job will not accept a 'COMMIT' revision parameter.

Also several modifications:

• allways apply Diff to master

• use a flake8-phab tox environment if it exists to run flake8; this is expected to produce phabrictor compliant lint results in a .phabricator-lint file

• activate the 'Process lint results from a file' flag

• include .phabricator* in archived artifacts.