fish_indent - Don't force single-command variables to be on the same line

This:

function foo
    first_long_variable=something \
    second_long_variable=something_else \
    echo $first_long_variable $second_long_variable
end

is obviously nicer than this:

function foo
    first_long_variable=something second_long_variable=something_else echo $first_long_variable $second_long_variable
end

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