We have a problem when in this situation, we checkout |patch| and rebase it on
top of |base|, thus including an |extra commit| that we shouldn't.

o master
|
. o patch
|/
o extra commit
|
o base (what gclient synced src at)

This does merge-base between |patch| and |master|, and cherry-picks only the
changes belonging to the patch.

Bug: 850812
Change-Id: Id09ae1682e53b69ed49b2fb649310de6a6a8a29e
Reviewed-on: https://chromium-review.googlesource.com/1098228
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>