[feature request] find git commit log before rebase

Assuming I have branches master (m), and a side branch (b), with a history tree like below:

m0 --- m1 -- m2 -- m3 -- m4 --- master (m)
        \          /      \
        b1 ------ b2       b3 -- b4 -- branch (b) (HEAD)
                  |
      (tag:POINT_BEFORE_REBASE)

The history of branch b is can be described as:

1. branch b is forked at point of m1
2. branch b is merged to master into m3,
3. branch b then is rebased (fast-forward) from b2 to m4
4. then branch b started its new life as b3 b4 after rebase

With the following command: I can find b3 and b4 since last fork-point

    git log --oneline $(git merge-base --fork-point master)..b

But how to find out commits b1 b2, given the fact that b2 is the point before rebase?

I understand it can be achieved via:

    git log --oneline m2..b2

That's because I know b2 is the point before rebase, and m2 is another child of the subsequent merge commit m3

I wonder how to do this with an simple (enough) command without me looking through the history and find b2 and m2 manually?

Thanks!