git-codereview: deflake TestSyncRebase

In order to guarantee different hashes on "client" and "server" we need to generate different
commit histories.

Fixes golang/go#10048.
Updates golang/go#9602.

Change-Id: I511f517639230d7aca74b3d2ce3bfa6a2c299a5c
Reviewed-on: https://go-review.googlesource.com/9951
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed