Subversion: Improve detection of svn directories

* Subversion was not found when calling the managesDirectory method
  in subversion's top level directory
* Subversion was not found for non-existing directories. This broke
  the wizard's version control support.

Task-number: QTCREATORBUG-7763
Change-Id: Iefbd8e179c48b188e4f079fce8f72ae8b3ec0236
Reviewed-by: Orgad Shaneh <orgads@gmail.com>