SVN: Fix project status command when no document open

The SVN project status command is using currentFileTopLevel() for the
working directory when it should be using currentProjectTopLevel().
If no document is open, currentFileTopLevel() will return an empty
string which means the current working directory of Qt Creator is used.

Change-Id: I848b1adf7166836c5d15e88796af77f8700a43f2
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>