This is a reland of e86fe982

Original change's description:
> bot_update: Set user.{name,email} when applying patch refs.
> 
> git rebase complains when user name/email has not been configured, as was
> the case for the Chromium trybot.
> 
> Bug: 643346
> Change-Id: Iab7dac0a98608e09b63a53d5bacb98925701fbfd
> Reviewed-on: https://chromium-review.googlesource.com/1031524
> Reviewed-by: Aaron Gable <agable@chromium.org>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 643346
Change-Id: I4b4d07675222d39d704560ed84f9c221e87aba8b
Reviewed-on: https://chromium-review.googlesource.com/1036309Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>