add debbisect

I cleaned up a script that I usually use to figure out which package and which version of it introduced a problem using snapshot.debian.org.

So this is a solution to the problem explained in https://wiki.debian.org/BisectDebian so @pabs might find this interesting.

Have a look at the man page or --help to see how to use it. Basically you give it a known good timestamp and a known bad timestamp and it will figure out which timestamp was the last good and which one is the first bad by bisecting snapshot.debian.org. The definition of what is good and what is bad comes from a script that you pass as the third argument and which will return with a non-zero exit status on failure. A apt cacher is started in the background to reduce the load on snapshot.debian.org, especially when the script is run multiple times on the same problem.