Default to external diff?

[behdad]

fdiff is great!

Internal python diff is way too slow on large strings. Maybe default to external unix diff command if available? That one is instantanous.

[behdad]

Eg. for comparing two fonts which are similar except for a couple lines, with ttx that is 53MB, fdiff by default takes ... well, I interrupted it after 4 minutes.

With external diff it takes 5s, most of which is the conversion to ttx. Diffing the ttx'es using unix diff takes only .3s.