Issues with Git when --preserve-merging and deleting a merge commit.

gistfile1.txt

git rebase -i --preserve-merges X

before:
                              master, HEAD
                                v
X---*---*---m1-------------m2---*      // rebase to delete spurious merge commit m1
         \ /  \            /
          a1   b1---b2---b3

expected:
                              master, HEAD
                                 v
X---*---*---a1--------------m2---*
              \            /
               b1---b2---b3

actual:
                    master|REBASE-i, HEAD
                           v
X---*---*---a1-----------(???)
              \            /
               b1---b2---b3