How my Git repository got corrupt in the first place
Ok, so how my Git repository got corrupt was (sort of) entirely my own fault. And why it was a problem was definitely entirely my own fault.
I had two stashes in my repository and decided for no apparent reason to pop the second stash straight after popping the first stash. Git told me there were conflicts in the second pop. I decided that I shouldn't really have popped them straight after each other (lesson #1 here) and decided to restore a Time Machine backup to get to the state where both stashes were still stashed. And that's …continue.