Suppose you are working on a large open source project with 20+ people. Suppose one of these people get mad and decide they want to sabotage the entire project.
My question:
What measures (if any) could be taken to prevent loss of code and information, in the event of an internal sabotage?
This is part of the purpose of version control systems, like git.
When someone makes a new feature, or changes any code at all in fact, they have to commit it to the project. It's a bit like editing here on Stack Exchange: when you edit your post, an item is created in the post's history showing the changes.
If someone sabotages your post (or in this case, project), then you can simply rollback the changes, setting your project back to the state it was in before the change.
The other method is of course backups. If for some reason this disgruntled individual deletes the entire project, backups are your recourse. Taking regular backups means you have a record, kept separately from the project, of the stages of development. You can then simply restore a damaged or deleted project from these backups.
Review and testing. That's all we got. If you want to be very careful, Review then Commit (RTC), so that someone affirmatively takes responsibility for each change as it becomes part of the main line.
Your question uses a very broad term, sabotage. Sneaking exploits into code is only one way that someone can try to be damaging to a project. Badmouthing, for example, is a whole other area. Building a strong community is the only defense, and meaningful code review comes from either a community or a BDfL.
