Skip exporting hg tags whose names are not valid as git tag name

When converting a mercurial repo to a git repo as in the README:

    $ mkdir git-repo; cd git-repo; git init; cd ..
    $ cd hg-repo
    $ hg bookmarks hg
    $ hg push ../git-repo

if the original repository contains tags which names are not valid as git tag name (e.g. names containing '*'),
the push fails with dulwich error caused by an invalid git tag name.

This patch will allow hg-git to skip those tags and complete rest of the process.