This assumes that the module's code is stored on github,

git diff will work on any git repository, regardless of where it is hosted; and, more generally, diff will work even if version control is not involved, if you can put the two source trees you want to compare side by side on the filesystem.

I'm not sure this is really what was asked for, though. If a function block has been moved and possibly changed, diff mostly shows you that it was moved, and you need to actually look at the moved code line by line to see if it also changed.