rebaseing topic branch which has merges from master

Hi,

I'd like to achieve the following:

---(BP)--- ....---(M1)---     ...    ---(M2)---   .... master
     \              \                     \
      \              \                     \
      (T1)---(T2)---(T3)---(T4)---(T5)---(T6)--- .... topic
 
 
                                   ||
                                   ||
                                   VV
 
 
---(BP)---....---(M1)---     ...    ---(M2)---   .... master
                   \                     \_____________
                    \                                  \
                   (T1)---(T2)---(T3)---(T4)---(T5)---(T6)--- .... topic

However, when I run
  git checkout topic
  git rebase M1
I get a conflict somewhere around a merge commit down the road, and when I
gitk to check where I am, I see master commits (M2, M2^) that have changed
their (sha1) name. That shouldn't happen, afaiu git, so what's wrong? Is
this simply not supported? Can I flatten the merge commits somehow before
rebasing?

Thanks,
Marc