Jakub Narebski [off-list ref] writes:
By the way, while it is fairly easy to follow one file, it is hard
to follow directory or glob... and there is a trouble that one file
might come from two files (as concatenation for example; but I don't
think git can detect it with default values of rename detection
heuristics).
That's why Linus's proof-of-concept is based on the git-blame
engine.