z3c.repoexternals recursively retrieves subversion directory listings from the url or path and matches directories against a previous set of svn:externals
if provided then against regular expressions and generates
qualifying svn:externals lines. The defaults generate a set of
svn:externals for all the trunks in a repository and keeps them up
to date with the repository as new trunks are added when the
previous externals are provided thereafter.