Fix in prep_for_changes to not retry with old change.
Fix in prep_for_changes to not retry with old change.

Right now for non repo repositories we don't delete any old
work as part of clean up.  We should as this affects retries.

BUG=chromium-os:16909
TEST=unittests + x86-gen-pfq

Change-Id: I23869aff73482cf45e5d00f47b159d527c5b577d
Reviewed-on: http://gerrit.chromium.org/gerrit/3179
Tested-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>

chadversary/chromiumos.chromite,zhang0137/chromite,coreos/chromite,zhang0137/chromite,coreos/chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite