Abort any cherry-picks before applying the patch, so that if the bots are in a
bad state, we don't fail.

Original change's description:
> Reland "gclient_scm: Use cherry-picking instead of rebasing."
>
> The failures were caused by:
>  1 - When one change (call it #2) has been uploaded on top of another (#1),
>      and (#1) has already landed, git cherry-pick complains that the range
>      '<merge-base>..<change #2>' contains empty commits, since the contents
>      of (#1) are already present in the tree.
>  2 - We did not abort the cherry-picking when 'git cherry-pick' failed,
>      so a failure made all further CLs in that bot fail.
>
> This CL fixes it and prevents further regressions.
>
> 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>
>
> Bug: 850812
> Change-Id: I83f38d0a258df3f5cd89e277f0d648badff29a22
> Reviewed-on: https://chromium-review.googlesource.com/1139554
> Reviewed-by: Aaron Gable <agable@chromium.org>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 850812
Change-Id: Ic65bda67c792bd7af5ec013a62d9615d1498eb3a
Reviewed-on: https://chromium-review.googlesource.com/1142805Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>