Rebasing a Git merge commit

问题

TL;DR: git rebase --preserve-merges origin/master

Warning: starting with Git 2.18 (Q2 2018, 5 years later), git --rebase-merges will ultimately replace the old git --preserve-merges. See What exactly does Git's “rebase --preserve-merges” do (and why?)

– VonC

May 27 '18 at 19:30

回答1

There are two options here.

One is to do an interactive rebase and edit the merge commit, redo the merge manually and continue the rebase.

Another is to use the --rebase-merges option on git rebase, which is described as follows from the manual:

By default, a rebase will simply drop merge commits from the todo list, and put the rebased commits into a single, linear branch. With --rebase-merges, the rebase will instead try to preserve the branching structure within the commits that are to be rebased, by recreating the merge commits. Any resolved merge conflicts or manual amendments in these merge commits will have to be resolved/re-applied manually."

回答2

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

We'll start in the same state found in the original 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 of master, so cherry-picking won't work.

First of all, let's create the correct history:

 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 merged commit, we only need a nice history.

The history will look like this (ignoring master):

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

Let's get the 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 would only be conflicts from files changed between 8101fe3 and f5a7ca8, it doesn't include 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

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 the incorrect index, with our good index from master.

In the end we will have this 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
```