git-codereview: set user.name and user.email before tests

The tests should not assume that the user running the tests have
global user.name and user.email settings in ~/.gitconfig. This
applies especially to the builder machines.

Fixes golang/go#9342, and golang/go#9433.

Change-Id: Ic1c973b133586edfa6dce3a169b7512db113fcc6
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/2117
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>