record last_revision used to determine partial success before doing import

To determine whether a code import got all the revisions in a branch or stopped early because of a revision limit, we do roughly this:

doImport(source, target)
partial = target.last_revision() == source.last_revision()

However, if the import takes several hours, as it does sometimes, it's moderately likely that the last_revision of the source has now changed, which will give rise to an (essentially) spurious result of partially successful.

Cheers,
mwh