Treadmill script: revamp

Major revamp: instead of stacking a vendor commit on top of
the treadmill changes, do it the other way around: vendor,
then apply treadmill diffs.

Reason: the build-all-new-commits test. Sigh. It fails in the
common case where our treadmill changes include a new struct
element in cmd/podman/images/build.go

Why this is good: well, superficially, it's more intuitive.

Why this is horrible: omg the rebasing games are a nightmare.
When the vendor commit is on top (HEAD), it's ultra-trivial
to drop it, rebase the treadmill changes on main, then add
a new vendor-buildah commit on top. As you can see from the
diffs in this PR, treadmill-as-HEAD introduces all sorts
of complex dance steps in which things can go catastrophically
wrong and you can lose all your treadmill patches. I try very
hard to prevent this, and to offer hints if there's a problem,
and heck in the worst case it's still git so it's still possible
to find lost commits... but it's still much riskier than the
old way.

Alternative I considered: using sed magic to disable the
build-all-new-commits test. So tempting... but that would
also disable the bloat check.

Signed-off-by: Ed Santiago <santiago@redhat.com>

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