Add ability to tag at a particular tag/commit

[cvan]

This would allow us to create a tag from an existing tag:

$ python tagz.py -r mozilla/fireplace -c create -t 2014.02.11-2 -s 2014.02.11

[robhudson]

Oh yeah, I needed to do this today to cherry-pick a fix onto an existing tag. This would be nice. 👍

[robhudson]

Re-reading your description perhaps my use-case is different. I need to cherry-pick a commit onto an existing tag and tag it as a new tag.