tests: add script to disable a selected wire protocol capability
This can be used to test that the client and server both fall back to the
previous wire protocol when the capability is not supported.
It is currently used by test-push-http.t and I plan to use it for the HTTP
long argument support tests.
http://mercurial.selenic.com/bts/issue436
$ hg init
$ hg -v log -v
$ hg -v log -v x