tests: prepare get-with-headers.py for MSYS get-with-headers.py took the http GET parameter as a command line parameter that had to start with '/'. MSYS on windows will mangle such paths. Instead of applying a workaround everywhere (such as an extra '/') we let get-with-headers.py add the mandatory '/'. That is consistent with the url path handling in the Mercurial url class. A few tests sent 'GET ?cmd=...' which is invalid. They will now send 'GET /?cmd=...'. This will not enable any tests for being run on windows - only remove one reason they were disabled.