Rebasing a branch that contains merges

Hello,

I've done some little experiments with rebasing a branch with merges on it. I wanted to check that what I found is what is intended.

I made a repository like this:

 * -- * -- * (master)
      |\
      | A (branch1) -- M (merge)
       \              /
        B (branch2) --

It's a horribly unfair thing to do, but while on merge I did

 git rebase master

I had been hoping that both branches would rebase, but only one did.  I got:

 * -- * -- * (master) -- B -- M (merge)
      |\
      | A (branch1)
       \              
        B (branch2)

"M" effectively now contains all of branch1's changes. If I hadn't referenced the merge head separately from the two branch heads, all the history in branch1 would now be represented by "M".

I don't think this is the right result.  Either
 * git should refuse to rebase because it would lose history
 * git should rebase both branches

The ideal result (perhaps unrealisticly) would have been

 * -- * -- * (master)
           |\
           | A (branch1) -- M (merge)
            \              /
             B (branch2) --

This is an unreasonable thing to expect a version control system to be able to do - it's a nasty, complicated wish. git is its own worst enemy for things like this; it's getting to be so good at nasty unreasonable things that they don't seem like fantasy-land wishes.

A perfectly valid answer is "don't be so ridiculous Andy, go away". :-)

Andy