I've downloaded the tarball and get to step 18.104.22.168 but my webserver starts throwing 500 errors when I try to access the site.
included with Bugzilla that will automatically do all of the database migration for you.
The larger the jump you are trying to make, the more difficult it is going to be to upgrade if you have made local code customizations.
Upgrading from 4.2 to 4.2.1 should be fairly painless even if you are heavily customized, but going from 2.18 to 4.2 is going to mean a fair bit of work re-writing your local changes to use the new files, logic, templates, etc.
Bugzilla is now hosted on Github, but we used to be hosted on git.