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>