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.