[git] Rebasing a Git merge commit

Take the following case:

I have some work in a topic branch and now I'm ready to merge back to master:

* eb3b733 3     [master] [origin/master]
| * b62cae6 2   [topic]
|/  
* 38abeae 1

I perform the merge from master, resolve the conflicts and now I have:

*   8101fe3 Merge branch 'topic'  [master]
|\  
| * b62cae6 2                     [topic]
* | eb3b733 3                     [origin/master]
|/  
* 38abeae 1

Now, the merge took me some time, so I do another fetch and notice that the remote master branch has new changes:

*   8101fe3 Merge branch 'topic'  [master]
|\  
| * b62cae6 2                     [topic]
| | * e7affba 4                   [origin/master]
| |/  
|/|   
* | eb3b733 3
|/  
* 38abeae 1

If I try git rebase origin/master from master, I'm forced to resolve all conflicts again, and I also lose the merge commit:

* d4de423 2       [master]
* e7affba 4       [origin/master]
* eb3b733 3
| * b62cae6 2     [topic]
|/  
* 38abeae 1

Is there a clean way to rebase the merge commit so I end up with a history like the one I show below?

*   51984c7 Merge branch 'topic'  [master]
|\  
| * b62cae6 2                     [topic]
* | e7affba 4                     [origin/master]
* | eb3b733 3
|/  
* 38abeae 1

Ok, that's an old question and it already have accepted answer by @siride, but that answer wasn't enough in my case, as --preserve-merges forces you to resolve all conflicts second time. My solution based on the idea by @Tobi B but with exact step-by-step commands

So we'll start on such state based on example in the question:

*   8101fe3 Merge branch 'topic'  [HEAD -> master]
|\  
| * b62cae6 2                     [topic]
| |
| | * f5a7ca8 5                   [origin/master]
| | * e7affba 4
| |/  
|/|   
* | eb3b733 3
|/  
* 38abeae 1

Note that we have 2 commits ahead master, so cherry-pick wouldn't work.

1. First of all, let's create correct history that we want:

   git checkout -b correct-history # create new branch to save master for future
   git rebase --strategy=ours --preserve-merges origin/master
   

   We use --preserve-merges to save our merge commit in history. We use --strategy=ours to ignore all merge conflicts as we don't care about what contents will be in that merge commit, we only need nice history now.

   History will looks like that (ignoring master):

   *   51984c7 Merge branch 'topic'  [HEAD -> correct-history]
   |\  
   | * b62cae6 2                     [topic]
   * | f5a7ca8 5                     [origin/master]
   * | e7affba 4
   * | eb3b733 3
   |/  
   * 38abeae 1
   

2. Let's get correct index now.

   git checkout master # return to our master branch
   git merge origin/master # merge origin/master on top of our master
   

   We may get some additional merge conflicts here, but that's would only be conflicts from files changed between 8101fe3 and f5a7ca8, but not includes already resolved conflicts from topic

   History will looks like this (ignoring correct-history):

   *   94f1484 Merge branch 'origin/master'  [HEAD -> master]
   |\  
   * | f5a7ca8 5                   [origin/master]
   * | e7affba 4
   | *   8101fe3 Merge branch 'topic'
   | |\  
   | | * b62cae6 2                     [topic]
   |/ /
   * / eb3b733 3
   |/  
   * 38abeae 1
   

3. The last stage is to combine our branch with correct history and branch with correct index

   git reset --soft correct-history
   git commit --amend
   

   We use reset --soft to reset our branch (and history) to correct-history, but leave index and working tree as is. Then we use commit --amend to rewrite our merge commit, that used to have incorrect index, with our good index from master.

   In the end we will have such state (note another id of top commit):

   *   13e6d03 Merge branch 'topic'  [HEAD -> master]
   |\  
   | * b62cae6 2                     [topic]
   * | f5a7ca8 5                     [origin/master]
   * | e7affba 4
   * | eb3b733 3
   |/  
   * 38abeae 1
   

Given that I just lost a day trying to figure this out and actually found a solution with the help of a coworker, I thought I should chime in.

We have a large code base and we have to deal with 2 branch heavily being modified at the same time. There is a main branch and a secondary branch if you which.

While I merge the secondary branch into the main branch, work continues in the main branch and by the time i'm done, I can't push my changes because they are incompatible.

I therefore need to "rebase" my "merge".

This is how we finally did it :

1) make note of the SHA. ex.: c4a924d458ea0629c0d694f1b9e9576a3ecf506b

git log -1

2) Create the proper history but this will break the merge.

git rebase -s ours --preserve-merges origin/master

3) make note of the SHA. ex.: 29dd8101d78

git log -1

4) Now reset to where you were before

git reset c4a924d458ea0629c0d694f1b9e9576a3ecf506b --hard

5) Now merge the current master into your working branch

git merge origin/master
git mergetool
git commit -m"correct files

6) Now that you have the right files, but the wrong history, get the right history on top of your change with :

git reset 29dd8101d78 --soft

7) And then --amend the results in your original merge commit

git commit --amend

Voila!

It looks like what you want to do is remove your first merge. You could follow the following procedure:

git checkout master      # Let's make sure we are on master branch
git reset --hard master~ # Let's get back to master before the merge
git pull                 # or git merge remote/master
git merge topic

That would give you what you want.

• From your merge commit
• Cherry-pick the new change which should be easy
• copy your stuff
• redo the merge and resolve the conflicts by just copying the files from your local copy ;)