[MERGE] Add Tags.rename_revisions().

[Jelmer Vernooij]

Jelmer Vernooij wrote:
> The attached patch makes us provide a rename_revisions() utility method
> on the Tags object. This method is mainly useful for dpush
> implementations, which need to update the tags in the local branch after
> the identity of some of the revisions that were pushed has changed.
Actually, not for dpush (as that uses Branch.pull(overwrite=True)), but
for svn-upgrade.

Cheers,

Jelmer