Git: redo merge with a different branch

Question

I have a long-running git fork and I wanted to merge the upstream branch. Unfortunately I did git merge upstream-topic and spent hours resolving merge conflicts before realizing I really wanted to do git merge upstream-master .

The branches have 99% the same content, except upstream-topic has a bunch of merge commits from merging upstream-master that I'd prefer not to have cluttering the history forever. Is there any way to "redo" the merge with upstream-master without losing all my conflict resolution?

I've just discovered git rerere and really wish I'd had it enabled :(

Answer 1

If the two branches upstream-topic and upstream-master really contain the same content and only differ in those extra merge commits, then you can simply reuse the content of your merged upstream-topic to solve the merge conflicts in upstream-master :

# save the current master which merged upstream-topic
git branch merged-topic

# reset master to its original commit
git reset --hard origin/master

# do the merge, getting lots of conflicts
git merge upstream-master

# instead of solving those conflicts again, just use all the contents
# of your already merged topic
git checkout merged-topic -- .

# check the status, resolve the conflicts, and commit
git status
git add -u .
git commit