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

Sergey Organov <sorganov@gmail.com> writes:

Perhaps. You would have to be careful though, so let me think aloud a bit...

Suppose you started from an upstream history whose tip was B, and you worked on merging some changes X an Y you made earlier on a side branch.

         X---Y
        /     \
   O---A---B---Z---

In the meantime, the upstream history has advance and now it looks like this:

   O---A---B---C---D

You want to forward-port the change done by X, Y on the side branch and its merge Z on top of D, right?

In other words, you want to have this:

         X-----------Y
        /             \
   O---A---B---C---D---Z'

In this case, replaying the difference going from B to Z on top of D may be better than redoing a merge between Y and D, in that the former will carry evil merges and resolution of conflict forward.

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.