Rebase a merge commit

Question

Suppose my Git repository initially has two branches: Foo and Bar.

... ─ Foo

... ─ Bar

I create a third branch, FooBar in which I commit the merge of the two other branches.

... ─ Foo ──┐
            FooBar
... ─ Bar ──┘

FooBar is now one commit ahead of both Foo and Bar. Next, I do some more work, committing a few times on Foo only.

... ── A ───┬── B ── C ── D ── Foo
            FooBar
... ─ Bar ──┘

The question is: since the first parent of branch FooBar is no longer Foo, can I rebase the merge commit in branch FooBar to again have Foo and Bar as its two parents? In other words, can I incorporate the development in Foo into the previously merged FooBar along with the unchanged Bar?

... ── A ── B ── C ── D ── Foo ──┐
                                 FooBar
... ─ Bar ───────────────────────┘

Answer 1

I realize this is quite an old topic but since I found this question while facing a similar situation I could as well tell what I used in the end.

git checkout FooBar
git rebase -p --onto Foo A

I tried to use the names from the original question. Note especially that A means the commit that is marked as A in the ascii art. You would probably use the hash of the commit instead or any other way to point to the commit where branches Foo and FooBar originally separate.

The -p flag is also known as --preserve-merges and it does pretty much what it says.

Note that the branch Bar doesn't play a role here so that line of commits might be a named branch or then not - it makes no difference.

Answer 2

You couldn't rebase under branch FooBar without changing what defines FooBar. What you could do is merge FooBar and Foo. That would have the same contents that you desire.