Comparing commits on two different branches

Hi,

I have two branches - I know that one differs from the other entirely
by a (large) number of commits that I have made over some time.
Unfortunately, these are now all mixed up with a load of other commits
that have been merged into both from the upstream project.

Is there any way that I can get the list of commits that represent the
difference between the two branches? Even better is there some way
that I can apply all those commits to a third branch (a newer version
of the project) without having to do hundreds of cherry-picks?

Thanks!