New tags not considered changes

After pulling the repository only a new commit will trigger the `then` commands
https://github.com/abiosoft/caddy-git/blob/master/git.go#L81

This behaviour prevents to check commit coverage by signed tags if the tags were added later on. To understand my use case: https://carmebar.github.io/2016/10/29/making-a-mirror-of-a-gitbased-blog.html#preparing-caddy

I propose to check for a change in tags too.

Thanks for your work.