Differential output only calculates diff correctly for leaves (most specific frames)

[itamarst]

Consider a diff of these two (cargo run --bin inferno-diff-folded old1.prof new2.prof | cargo run --bin inferno-flamegraph -- - > out.svg):

parent;first_child 10
parent;second_child 10
parent2 30

and

parent;first_child 30
parent;second_child 30
parent2 30

The resulting flamegraph shows parent as unchanged, even though it's actually gotten larger: