scm.add() doesn't work properly with svn 1.7

scm.add() doesn't work properly with svn 1.7
https://bugs.webkit.org/show_bug.cgi?id=86779

Reviewed by Ojan Vafai.

The code relied on '.svn' directories being present
in order to tell if a directory had already been added to the
repository; this is no longer true in SVN 1.7.

* Scripts/webkitpy/common/checkout/scm/scm.py:
(SCM):
(SCM.in_working_directory):
* Scripts/webkitpy/common/checkout/scm/svn.py:
(SVN):
(SVN.in_working_directory):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@117526 268f45cc-cd09-0410-ab3c-d52691b4dbfc