Script to backdate tags

After importing one of my projects from Arch, I wanted to add tags that
indicated its major releases.  Unfortunately, these tags for older
releases would show up before the more recent releases in the gitweb
output.  I searched in vain for a way of backdating tags, and finally
decided to make a script to do this for me.  Here it is.

This may run into issues if someone uses the "\" character in their tag
names, but I didn't want to bother fixing this until it was affirmed
that this script would be considered generally useful.