Fixes T11940. In 2.11.0, Git has made a change so that newly-pushed changes are held in a temporary area until the hook accepts or rejects them.
This magic temporary area is only readable if the appropriate GIT_ENVIRONMENTAL_MAGIC variables are available. When executing git commands, pass them through from the calling context.
We're intentionally conservative about which variables we pass, and with good reason (see "httpoxy" in T11359). I think this continues to be the correct default behavior.