If the server had an internal error and returned 500, there's nothing
to read, so "response.read()" blocks indefinitely. Only output the
response if there's really a response.
Fixes HTTP protocol violation introduced in e4a5f5db7028. 'hg serve' would show
a stacktrace when loading pages that not had been modified.
There was test coverage for this, but the wrong response headers wasn't shown
and thus not detected.
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.
