When updating the CIPD client to be identified by SHA256 hash, old clients (that
have no idea about SHA256) fail during 'selfupdate'.

We'll roll our SHA256 support in two stages:
  1. Deploy new client that understand SHA256 using its SHA1 name, so
     self-update from old clients works.
  2. Deploy same (or newer) client using its SHA256 name. This would work since
     the client doing the self-update already understands SHA256 at this point.

But we can't guarantee that ALL depot_tools deployments will update through
stages (1) and (2) sequentially. Some of them may skip (1) and end-up directly
in (2), failing on 'selfupdate'.

This CL makes sure they can recover from this state by rebootstraping the client
from scratch (this works with SHA256 hashes).

R=nodir@chromium.org, iannucci@chromium.org
BUG=821194

Change-Id: I27dece19e0305b5b2d6f8b0130631c1bf5f6499c
Reviewed-on: https://chromium-review.googlesource.com/1149454
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>