This isn't an answer how to use jigit, but I think you're using the wrong tool for the job here.
jigdo (and it's associated tools like jigit) is useful to people who have an Ubuntu mirror handy and want to assemble an ISO out of that archive. I've done this and it's kind of annoying, it's more a tool for people who run archives and want to make it available to jigdo users.
It sounds like you want to save bandwidth and so you don't have to redownload things in the ISO. The end user tool you want to use for that is zsync. In the ISO directory of ubuntu releases (and mirrors) you will find a .zsync file that you can use to keep your ISO up to date. You use it like this:
Will keep an up to date ISO of the alternate installer for precise for you. The next day you just run that again and it will sync just the differences between the 2 ISOs.
The testing tool testdrive is a convenient tool for testing development releases in a VM; it has built in support for zsync downloading. You can use testdrive to make getting the ISOs easier.
It has downloading functionality built in, which will ask you which ISO you want and then keep an updated copy for you in
~/.cache/testdrive/iso, and the GTK gui will automatically notify you when new ISOs are available or you can just use the command line version.