Don't apply delta to enormously large buffers

[jumper047]

Introduces new variable - magit-delta-max-lines, set to 1000 by default. If buffer lines exceeded this value, then default magit highlighting used instead of delta to keep emacs responsive. Regarding default value - 1000 just works for me:). It tooks 2-3 seconds to display diff on Core-I7, so maybe it should be decreased. This MR fixes #26

[dandavison]

Thanks! I'm running your commit locally. I might be slightly slow to review and merge due to day job.

[dandavison]

If buffer lines exceeded this value, then default magit highlighting used instead of delta

I'm not seeing any color when it disables delta. Do you see the usual magit colors?

[daanturo]

Do you see the usual magit colors?

I still can see Magit's normal red and green colors, for example this commit: https://github.com/dandavison/delta/commit/79979c272d6daa876ffb3dbad7a6550c22155c8c