Replacing a git merge with another?-CodePudding

I have a branch with commits that look like this:

A->B->C

A and B are merges from other branches which have not yet been merged to master. C contains the relevant changes to this feature branch.

C is dependent on A and B. A just got changed dramatically. However, these changes do not effect the parts of the code C depends on. I would like to replace the merge from A with a merge from the new A.

In this case, the new changes are based on top of A's old head. But I'd like to know how to do this in the case that new commits are not children of old ones.

Said another way, I constructed the branch by:

Checking out master
Merging A
Merging B
Making changes and committing them as C.

I could create the branch I want now by doing exactly those things again (with the updated A), but I'd like to be able to just replace the A merge without reconstructing the branch.

CodePudding user response：

You could play a trick to try to avoid having to redo all the merges you have done (including A) but you will be rewriting history, there's no way to avoid it.

git checkout -b hack A # create a _hack_ branch where we will first get the _real_ contents of the new A, B and C
git merge new-A # merge "the new A" into hack
git merge B # merge B
git merge C # merge C
# this branch now has 3 commits that have _the contents_ of the _new_ A, B and C.... it's time to hack history to get this in
git commit-tree -p A~ -p the-new-A -m "The comment for the new A" hack~2^{tree} # create a replacement commit for A
# last command will print an ID, and we will use it in the next command
git commit-tree -p commit-id-from-previous-command -p B^2 -m "Comment for B" hack~^{tree} # create a replacement commit for B
# will print a commit id, will use it in next command
git commit-tree -p commit-id-from-previous-command -p C^2 -m "Comment for B" hack^{tree} # create a replacement commit for C
# again, we get a commit ID which should be the branch like what you wanted.

Do a git checkout, check the contents and also check its history (git log, gitk).... if you like it, then git branch -f master the-commit-id # or no commit ID if you did a checkout of it git checkout master

Of course, you have rewritten history which means that it has consequences so use with care.