Let me explain the problem in detail.

I have a main git branch on which I created a new side branch bug10101010, now I wan't to merge the bug10101010 to main. So far everything is good.
Now I have a different branch of the same product, named legacy. I wan't to merge the bug10101010 to the legacy branch in GIT.

Any ideas?

I can't just merge it directly, as the branch bug10101010 is spin off from the main branch and in the legacy I need only the diff between the branch bug10101010 and its parent branch.