convert: add tagmap option Tests have been updated.

convert: add tagmap logic Previously, there was no way to rewrite tags on the fly while converting. Now, we add similar logic to branchmap to provide a way to map old tags to new tags. Currently, this is not enabled since there is not yet a command-line option.

convert: avoid updating tags when there is nothing new Previously, when converting from a mercurial repo there would be an extraneous commit at the end of the convert process that would rewrite tags. Now, we check if there are any new tags before doing this rewriting.