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.
