remote-curl: hoist gzip buffer size to top of post_rpc

When we gzip the post data for a smart-http rpc request, we
compute the gzip body and its size inside the "use_gzip"
conditional. We keep track of the body after the conditional
ends, but not the size. Let's remember both, which will
enable us to retry failed gzip requests in a future patch.

Signed-off-by: Jeff King <peff@peff.net>