convert: fix bug of wrong CVS path parsing without port number (issue3678)

The cvsps.py:getrepopath suffers from a string parsing bug (it returns
"user@server/path/to/repository" if the CVSROOT is given like this:
":pserver:user@server/path/to/repository" ), which gives returnes the wrong
value becouse cvsps.py fails to strip the prefix from filenames.
With this patch for the same input we get the correct repo path that is:
"/path/to/repository"

--HG--
branch : stable
extra : rebase_source : 79ab87fc9ffab57584997372013533cc612f8636