Add a new histedit verb: graft. This allows grafting (copying) of
changesets from outside of the history that is being edited, using the
node hash of the other changeset.
The other changeset is left intact - its contents are duplicated in a new
changeset.
The other changeset must not be one of the changesets that are being
edited as part of the histedit.
At first I was confused by the new rev numbers here, but then I realized that in this test we're actually still stripping the old rev 3, thus the re-ordering of rev numbers.