Re: [PATCH] docs: Clarify what git-rebase's "--preserve-merges" does

> Junio C Hamano <gitster@pobox.com> writes:
>
>> I wonder if it will be the right way to get a correct result to
>> apply the difference to go from B to Z on top of an old commit when
>> you are side-porting.
>>
>> Imagine you want to backport the same X-Y history by redoing the
>> merge Z on top of another child of O (i.e. A's sibling).  That is,
>> you start from this:
>>
>>
>>          X---Y
>>         /     \
>>    O---A---B---Z---
>>     \
>>      M---N
>>
>> and would want to create this:
>>     
>>     
>>    O           X'--Y'
>>     \         /     \    
>>      M---N---A'--B'--Z'--
>>
>> As long as everything down to the merge-base of the parents of the
>> original merge (in this example, merge-base across Y and B that are
>> Z's parents, which is A) is being transplanted, "apply the
>> difference going from B to Z, on top of B', to obtain Z'" should
>> work, I would think.
>
> And just after I send the message because I needed to catch a bus, I
> notice that there is a problem.
>
> Actually, "replay diff going from B to Z instead of merging" must be
> done very carefully.  Imagine when Y in the original history were a
> cherry-pick of M.  What you would be creating would look more like
> this instead:
>      
>     O           X'--.
>      \         /     \    
>       M---N---A'--B'--Z'--
>
> because Y' becomes a no-op, as the transplanted history already has
> M applied.  But the original "diff going from B to Z" has the effect
> of M already in there.  You would end up adding the same hunk twice
> without noticing. You somehow need to come up with a way to deal
> with this.