Styled zero lines fix

With `interactive.diffFilter = delta --color-only` delta is called
by e.g. `git add -p`, but in this mode git hides the terminal from
the pager.

Plus/minus lines correctly use ANSI sequences to paint up to to the
end of the line, but zero lines always use spaces. This needs the terminal
width, but it is not available for diffFilter. So the fallback
of 80 is used, and zero styles did not extend to the full terminal width.

Since zero lines are only rarely styled (e.g. via
`--zero-style='syntax "#1d1f21" dim'`), this was never noticed.

This also crashed delta when a zero line was longer than 80.