Test manifest changes and chromite changes in the Pre-CQ.
Test manifest changes and chromite changes in the Pre-CQ.

The Pre-CQ will now commit manifests to a temporary directory
and check out the repository from them to see if they work. This
ensures that manifests are fully tested before they go into the
regular CQ.

Originally, when the Pre-CQ was written, it was not possible to
use --bootstrap in the Pre-CQ -- patches would fail to apply because
it tried to apply the same patch twice. This has since been fixed.

BUG=chromium:213079, chromium:265997
TEST=Verify bad manifest changes are rejected.
TEST=Verify chromite changes are tested better now too.

Change-Id: Ifcf4489040aaec22ac8ecc198e17ecc28ac1f795
Reviewed-on: https://chromium-review.googlesource.com/182932
Reviewed-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

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