insecure use of os.system()

[Luke Faraone]

SYNOPSIS:
        If a user was tricked into using the "Command Shell" menu item
        while in a directory with a specially-crafted name,
        svn-workbench would execute arbitrary commands with the
        permissions of the user.

STEPS TO REPRODUCE:
     1. Add "https://github.com/lfaraone/turbulent-octo-garbanzo" as a
        project in svn-workbench
     2. Checkout the project
     3. Navigate to "trunk/$(xeyes)"
     4. Click "Actions", then "Command Shell"

The `xeyes` program (if installed on your system) should start.

Source/wb_shell_unix_commands.py starting at line 53:
        def ShellOpen( app, project_info, filename ):
            app.log.info( T_('Open %s') % filename )
            cur_dir = os.getcwd()
            try:

        wb_platform_specific.uChdir( project_info.getWorkingDir() )
                os.system( "xdg-open '%s'" % filename )
            finally:
                wb_platform_specific.uChdir( cur_dir )

The code should instead start a subprocess in a secure way, such as
using subprocess.call().