Use self._filesystem in more places in the SCM classes

Use self._filesystem in more places in the SCM classes
https://bugs.webkit.org/show_bug.cgi?id=79415

Reviewed by Adam Barth.

* Scripts/webkitpy/common/checkout/scm/detection_unittest.py:
(SCMDetectorTest.test_detect_scm_system):
* Scripts/webkitpy/common/checkout/scm/git.py:
(Git.find_checkout_root):
(Git.to_object_name):
* Scripts/webkitpy/common/checkout/scm/scm.py:
(SCM.__init__):
(SCM.in_working_directory):
* Scripts/webkitpy/common/checkout/scm/svn.py:
(SVN.find_uuid):
(SVN.find_checkout_root):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108693 268f45cc-cd09-0410-ab3c-d52691b4dbfc