This should be one of the core features of Git, but for some reason it's impossible to figure out how to search for a string in your commit history and see the diffs that that string is in. Here's the best I've come up with:
To find which commits and which files a string was added or removed in:
git log -S'search string' --oneline --name-status
To see the diff of that
git log -S'search string' -p | grep 'search string' -C5
You can also do a regex search by using -G instead of -S.