git.loader: Migrate away from SWHConfig mixin

Related to swh-core#1532 (closed) swh-core#1410 swh-core!159 (closed)

Test Plan

tox

and docker:

time doco exec swh-loader swh loader run git https://github.com/huge-success/sanic.git
INFO:swh.loader.git.BulkLoader:Load origin 'https://github.com/huge-success/sanic.git' with type 'git'
Enumerating objects: 143, done.
Counting objects: 100% (143/143), done.
Compressing objects: 100% (102/102), done.
Total 15402 (delta 78), reused 77 (delta 40), pack-reused 15259
INFO:swh.loader.git.BulkLoader:Listed 1072 refs for repo https://github.com/huge-success/sanic.git
{'status': 'eventful'}
docker-compose exec swh-loader swh loader run git   0.49s user 0.39s system 1% cpu 48.581 total

---

Migrated from D4128 (view on Phabricator)

swh/loader/git/tests/test_tasks.py

-# Copyright (C) 2018-2019  The Software Heritage developers
+# Copyright (C) 2018-2020  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 
-def test_git_loader(mocker, swh_scheduler_celery_app, swh_scheduler_celery_worker):
+def test_git_loader(
+    mocker, swh_config, swh_scheduler_celery_app, swh_scheduler_celery_worker
+):
     mock_loader = mocker.patch("swh.loader.git.loader.GitLoader.load")
     mock_loader.return_value = {"status": "eventful"}
 
@@ -20,7 +22,7 @@ def test_git_loader(mocker, swh_scheduler_celery_app, swh_scheduler_celery_worke
 
 
 def test_git_loader_from_disk(
-    mocker, swh_scheduler_celery_app, swh_scheduler_celery_worker
+    mocker, swh_config, swh_scheduler_celery_app, swh_scheduler_celery_worker
 ):
     mock_loader = mocker.patch("swh.loader.git.from_disk.GitLoaderFromDisk.load")
     mock_loader.return_value = {"status": "uneventful"}
@@ -42,7 +44,7 @@ def test_git_loader_from_disk(
 
 
 def test_git_loader_from_archive(
-    mocker, swh_scheduler_celery_app, swh_scheduler_celery_worker
+    mocker, swh_config, swh_scheduler_celery_app, swh_scheduler_celery_worker
 ):
     mock_loader = mocker.patch("swh.loader.git.from_disk.GitLoaderFromArchive.load")