git-remote-entire: don't fail push on feeder errors after send-pack exited cleanly

handlePush waited on the feeder goroutine before sp.Wait(), and any non-nil feedErr aborted the push. But the feeder's io.Copy reads from resp's underlying TCP socket, which can drop after send-pack has already drained the full receive-pack report-status and exited 0 -- e.g. when httptest.Server.Close runs in a parallel CI test or an idle-conn reaper closes the keep-alive. The push was complete at the protocol level (ref committed server-side, send-pack happy, helper- status emitted) but the helper still surfaced "fatal: piping receive-pack response to send-pack: read tcp ...: use of closed network connection" and git exited non-zero.

c70d7cddf moved resp.Close into the feeder so main couldn't slam the body shut mid-Copy, but it didn't cover the externally-closed-socket case. Treat send-pack's exit code as the authoritative signal: if it exited 0, the push succeeded and any feeder error is cleanup noise; if it failed, surface both errors as before.