Currently gclient might include extra commits when applying patches.
For example, in this case 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 change uses the merge-base between |patch| and |master| to cherry-pick only
the changes belonging to the patch.

Bug: 850812
Change-Id: I138192f96bc62b1bb19b0e1ad952c8f8c67631c4
Reviewed-on: https://chromium-review.googlesource.com/1137052
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>