Subject: Use --version instead of obsolete -V to check for co's existence

The --use-rcs option uses 'co -V' to verify that co exists. However, recent
versions of rcs produce a warning on stderr to use --version instead; this
causes a false negative, causing --use-rcs to fail.

Author: Stephen Oberholtzer <stevie@qrpff.net>

--- a/cvs2svn_lib/rcs_revision_manager.py
+++ b/cvs2svn_lib/rcs_revision_manager.py
@@ -29,7 +29,7 @@
   def __init__(self, co_executable):
     self.co_executable = co_executable
     try:
-      check_command_runs([self.co_executable, '-V'], self.co_executable)
+      check_command_runs([self.co_executable, '--version'], self.co_executable)
     except CommandFailedException, e:
       raise FatalError('%s\n'
                        'Please check that co is installed and in your PATH\n'