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.
Many tests didn't change back from subdirectories at the end of the tests ...
and they don't have to. The missing 'cd ..' could always be added when another
test case is added to the test file.
This change do that tests (99.5%) consistently end up in $TESTDIR where they
started, thus making it simpler to extend them or move them around.
