🐛 diff -U0 rendered with lines out of order

[dandavison]

As reported in #259 (comment), delta is rendering the output of git diff -U0 incorrectly. cc @Amorymeltzer

For example:

• git --no-pager diff --color=always -U0 b2257cf~..b2257cf | less -RS
• git diff -U0 b2257cf~..b2257cf