Preserve merge changes with rebase -p

Hi all,

I needed to change the author of a commit that was before a merge in the history. Here is a figure of my situation, E being the commit I want to edit, and M the problematic merge commit:

     o---o---o---o---o---o---o---o  master
              \       \ 
               o---E---M---o---o  topic

In a simple case, git rebase -i -p from topic does the trick here, I can amend E and M is re-created correctly.

The thing is, my M was not a "simple merge commit", but contained some changes needed for the code in master and topic to work together. In that case, git rebase -i -p recreates a merge commit, but it does not keep the changes introduced in M.

Is there any way to keep the M's changes in my case (except to manually patch & amend the re-created merge commit) ? If not, maybe it would be a nice functionnality to implement ?

Cheers,
  Matteo

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