failure rebasing a merge

  status triaged
  importance high

In discussing this with Jelmer on IRC, it seems that when rebase is trying to re-apply the merge, it is re-applying something that is already in the ancestry. For example:

A
|\
B C
|\|
D E
  |
  F

If you rebase "F" on top of "D", then you get something like:

A
|
B
|\
D |
| |
C'|
|/
E'
|
F'

And it is a little weird to merge B into a descendent of itself. So what you
should really get is:

A
|
B
|
D
|
C'
|
F'

On the other hand if you have:

A-.
|\ \
B C D
| |/
E F

And you rebase 'F' onto 'E', then you *should* get:

A
|\
B D
| |
E |
| |
C'|
|/
F'

So I think when rebase comes to a node which is a merge, it should check if all
merged revisions are already present. If so, then it should just skip that
node, and move to the next. Otherwise do the same cherry-pick merge, of the
changes against the primary parent, and mark the merged revision id.