rr delta/diff

My apologies if this request is a little "out there", feel free to close it if so.

My use-case:
I have a bug.
I know which change caused this bug. (right down to the exact line)
However, this change causes tons of subtle ripple effects in the execution of my program, such that, by the time an assert trips, I am far away from the start of the problem.

What I want is to do 2 runs of RR, 1 before and 1 after the patch.

Then I want to run some kind of 'rr delta' program, which will tell me the earliest point at which the execution of the two traces begins to diverge.

Likely this will also need:
() some kind of 'ignore and continue' command to skip the boring stuff that is slightly out of order
() some kind of 'only consider these threads' as interesting
(*) some kind of 'ignore filter' to ignore whole swaths of source code where the differences are uninteresting.

Start with the proposed rr delta workflow: compare two RR runs and identify the earliest point where their traces diverge. Define how ignore-and-continue, thread selection, and source-code ignore filters should interact, and consider the work complete when the design specifies a usable divergence result and these filtering behaviors.