internal/requestlog: use http.Get instead of s.Client.Get

Same rationale as golang.org/cl/78539.

Change-Id: I32f0f412b3d31d803b554a0cc5df9265e7405796
Reviewed-on: https://go-review.googlesource.com/78541
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>