This CL propagates <diff_base> all the way to become parent commit
of the syntentic commit generated by squashing the current branch.
BUG=649846
R=agable@chromium.org
Change-Id: Ided7ebbb5c3a1114cac18adb62b3a9c27610018c
Reviewed-on: https://chromium-review.googlesource.com/475229
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>