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.

Reintegrate

@@ -173,7 +173,7 @@ no)
 			into="--into $original_branch"
 		else
 			original_branch=
-			into=
+			into=${INTO:+"--into $INTO"}
 		fi
 	}