Can Git repos be hacked or otherwise manipulated?

Hi,

Let's say you're working with a team of elite hackers, passing a tarball of a Git repo back and forth as you complete your mission. Now let's say one of them has malicious intent. What are the possibilities that he or she can, for instance, hide changes made to a script or binary that does something malicious if executed? Or perhaps maybe there are other such scenarios one should be made aware of?

Thanks and have a nice day!

--Jonathan

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