I need to search the history of a file in a git repository to find a line that is gone. The commit message will not have any relevant text to search on. What command do I use?

Further details: this is the history of my todo list out of our non-stellar task tracking software. I've been keeping it for two years because there's just not enough information kept for me in the software. My commit messages usually have only the task ids, unfortunately, and what I need to do is find a closed task by subject, not by number. Yes, the real solution is better task tracking software, but that is completely out of my hands.