Convert git stdin/stdout to and from bytes.
Convert git stdin/stdout to and from bytes.

Change-Id: Ifab14026db8d072e04651ff2bf2d3a4a7cbb0f4f
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/3404068
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Brian Ryner <388ea68edb4d8f934960c128120aee09a8f854bf@google.com>

luci/recipes-py,luci/recipes-py