This reverts commit c9121141.

Reason for revert: broke patch application on infra/config https://crbug.com/853032

Original change's description:
> gclient_scm: Use cherry-picking instead of rebasing.
> 
> 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>

TBR=agable@chromium.org,ehmaldonado@chromium.org

Change-Id: Ib3feeee2f44f5441713383f1dbf08db16fae4717
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 850812, 853032
Reviewed-on: https://chromium-review.googlesource.com/1101977Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>