(stable) Decode "Content-Encoding: gzip" content

Summary:
Fixes T10228. When we receive a gzipped request (rare, but git may send them), decode it before providing it to the application.

This fixes the issue with proxying certain requests described in T10228.

Test Plan:

• Applied this fix in production.
• Cloned a problem repository cleanly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10228

Differential Revision: https://secure.phabricator.com/D15145