git log -S<search> can be used to search for a string in Git, even if it was previously deleted. It's known as the 'pickaxe' option.
-p shows the diff too
-i make it case insensitive
Using -G over -S will show the result, even if the number of occurrences of the search term exists is unchanged from the commit. This is helpful to show when a line is moved, for example. -G also accepts a regex.
https://git-scm.com/docs/git-log#Documentation/git-log.txt---pickaxe-all