gitproto: tighten SpooledBody encapsulation and trim docs

Three small refinements:

• Encapsulate SpooledBody's request wiring as (*SpooledBody).applyTo, so PostRPCStreamBody doesn't reach into unexported size/path fields to build req.ContentLength + req.GetBody.

• Trim the multi-paragraph Cloudflare/idle-write narrative on PushObjects and PostRPCStreamBody. The story now lives once on SpooledBody (the type that actually captures the workaround); the other call sites point there.

• Fix the countingWriter comment to say why the counter is atomic (concurrent read from the progress ticker) rather than restating what the type does.