This reverts commit 2074a4b7.

Reason for revert: this is a reland, because it didn't actually break Angle.

Original change's description:
> Revert "bot_update: speculative fix for gerrit patch application failure."
> 
> This reverts commit 2b4da122.
> 
> Reason for revert: likely broke Angle http://crbug.com/692140
> 
> Original change's description:
> > bot_update: speculative fix for gerrit patch application failure.
> > 
> > bot_update doesn't properly cleanup git index state from unmerged paths,
> > which are likely if Rietveld patches were failed to be applied in prior
> > runs in the same checkout dir.
> > 
> > R=​machenbach@chromium.org
> > BUG=692067
> > 
> > Change-Id: I8fd326caefce8623731697a368cbad3159405257
> > Reviewed-on: https://chromium-review.googlesource.com/442426
> > Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
> > Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> > 
> 
> TBR=machenbach@chromium.org,tandrii@chromium.org,chromium-reviews@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=692067,692140
> 
> Change-Id: Iec808f28f3a0af99914a279536bb20a8975e02ba
> Reviewed-on: https://chromium-review.googlesource.com/442485
> Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
> Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
> 

TBR=machenbach@chromium.org,tandrii@chromium.org,chromium-reviews@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=692067,692140

Change-Id: I4b4b66fa74f11e73e066602a9e7efabd5a29a974
Reviewed-on: https://chromium-review.googlesource.com/443345
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>