fix: update unified diff highlight after toggling word diff

[rockyzhang24]

Closes #92

Problem:

When pressing r twice to toggle word diff followed by pressing d to open unified diff, state.prev_diff will be none and state.diff will be current. Both prev_is_word and is_word will be 0, so that if block to update syntax will be skipped.

Solution:

Update syntax if prev_diff is none.

Thank you!

[rhysd]

Nice catch. Thanks.

[rhysd]

I confirmed tests passed on my local. I'll fix the lint error later.