Meta/Reintegrate: allow preparing an update on a detached HEAD

Meta/Reintegrate: allow preparing an update on a detached HEAD

While waiting for build and test of the current round of
integration, we should be able to prepare the next round while on
a detached HEAD.  By running

    $ git co --detach master
    $ INTO=jch Meta/redo-jch.sh

we can prepare a sequence of "Merge branch 'foo' into jch" commits
that can be used to replace 'jch' that is currently being tested,
without disrupting the test.