More accurate elision

[abhinav]

Elision correlation between the - and + sections of a patch is very naive right now. It matches based on positions of the ellipses in the patch, so it can easily get things wrong.

We should consider a more sophisticated elision matching implementation; perhaps by diffing the - and + sections.

Alternatively, or in addition to that, we may want to support named elisions or equivalent for cases where even best-effort elision won't behave correctly.