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: I42a50dc890598dd3a3b6c626c6db70ffe06c14d3
Reviewed-on: https://go-review.googlesource.com/9256
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed