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This CL adds the --step flag to make debugging large rolls more
efficient.

Also adds the --apply-patch flag to give users the ability to apply
local patches before performing each build (not compatible with
--cloud). Note that this doesn't work for patches to supersize; we copy
supersize to a temp dir before modifying local state.

Side note: using diagnose_bloat.py with CLs before and after the blink
rename require "git config merge.renamelimit 20000".

Bug: 831601
Change-Id: Ida5d0a879355e48c942dab07afd5489c36bca1ca
Reviewed-on: https://chromium-review.googlesource.com/1011681Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550714}