Support simple redirect response with HTTP

Okay, so I'm somewhat embarrassed about this PR. :flushed:

I know this can't possibly be the right way to do this, but I started at the existing spot where the status code is checked and ran from there, attempting to recognize the 3xx response, update the environment and restart the connection.

I saw a relatively easy way to update the path, but no easy way to update the host from inside the callback, so I started out but just accepting redirects that don't change the host.

At first I just updated the path with the full new location, but since the redirect contains the combined path and service url, but we store those two pieces separately, and I saw no easy way to reliably split the redirect location into the two parts. Instead I just made it so that we only accept a matching service url and as soon as the request is complete, the code restores the original path.

I ran into some infinite loops while I was muddling around which is why I put the stall detection into `gitno_send()` (i.e. if we try to write some data enough times with zero bytes accepted, I'll abort the write loop). Once I got the rest of my problems worked out, I suspect the stall detection is unnecessary. I'm happy to rip that shit out if it is just stupid.

Anyhow, someone who actually understands the network code more deeply please take a look, have a good laugh, and propose an alternate fix that will allow for redirects.