convert: bail out in Subversion source if encountering non-ASCII HTTP(S) URL Before this patch, in the tested case, urllib raised `httplib.InvalidURL: URL can't contain control characters. '/\xff/!svn/ver/0/.svn' (found at least '\xff')`, which resulted in that the URL was never recognized as a Subversion repository. This patch adds a check that bails out if the URL contains non-ASCII characters. The warning is not overly user-friendly, but giving the user something to type into a search engine is definitively better than not explaining why the repository was not recognized. We could support non-ASCII chracters by quoting them before passing them to urllib. However, we would want to be compatible with what the `svn` command does, which converts the URL from the locale encoding to UTF-8, percent-encodes it and sends it to the server. If the locale encoding is not UTF-8, the behavior is IMHO not very intuitive, as the `svn` command may send different (percent-encoded) octets than what was passed on the console. Instead of copying this behavior, we better leave it forbidden.