This reverts commit fb78b368.

Reason for revert: https://bugs.chromium.org/p/chromium/issues/detail?id=864301

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

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

Change-Id: I57299e60e58eac5656dc88937c622d0d14c4ba37
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 850812
Reviewed-on: https://chromium-review.googlesource.com/1139553Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>