Pushing a sub-branch without rebasing

Question

Say we have this topology:

X--X--X--             origin/master
    \     
     \     
      \--X--X--X--             b1 (private - not to be shared)
             \
              \
               \--X--X--X--    b2 (to be shared upstream)

We want to share only b2 but not b1, so we can do:

git rebase --onto origin/master b1 b2
git push origin b2

Is it possible to achieve the same goal without using rebase and changing local history? (pushing b2 sans b1 commits)

Answer 1

No, that's not possible. Some b1 commits are in b2 's history, so you'll have to rewrite history to get rid of those.

Answer 2

It helps to think not of individual files, but of the commit objects as a whole. To get to b2, you need some of the commit objects from the second line.

A--B--C--                  master
    \
     \--D--E--F--          b1
            \
             \--G--H--I--  b2

In order to get the I commit, you need all the commit objects A, B, D, E, G, H, I.

So unless you rebase G from B you will not be able to exclude the D, E commits.