R1:5e3754828f7b

Fix handling of gzip in VCS responses

Summary:
Fixes T10264. I'm reasonably confident that this is the chain of events here:

First, prior to 8269fd6e, we would ignore "Content-Encoding" when reading inbound bodies. So if a request was gzipped, we would read a gzipped body, then give `git-http-backend` a gzipped body with "Content-Encoding: gzip". Everything matched normally, so that was fine, except in the cluster.

In the cluster, we'd accept "gzip + compressed body" and proxy it, but not tell cURL that…