review: handle hosting Gerrit in a sub-path

The git-review module is useful for other gerrit sites. Some
host gerrit under a sub-path. Add tests for both googlesource.com
repos and non-googlesource.com repos.

Change-Id: If2848128d6957db61ac50cbab7e6927da431002b
Reviewed-on: https://go-review.googlesource.com/20553
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
3 files changed