Git: restore my changes which were amended to merge commit

You have 2 SHAs that are of interest here - the original merge commit, and the amended merge commit. What you want to do is git reset your HEAD to the original merge commit, while preserving your index and working directory. You can then create a new commit hanging off the merge commit.

Use

git reflog

to find the SHA of the original merge commit

reset to the commit with git reset ORIGINAL_MERGE_COMMIT_SHA or directly from reflog with git reset HEAD@{X} where X is 1 or the position in the reflog that represents the merge commit.

You should now be ready to git commit your original changes and don't pass in --amend here and you will create a new commit.

I've found one way which works:

git diff HEAD~1 > p.patch
git checkout master
git checkout -b branch-name

Manually edit p.patch to remove unrelated changes from merge.

git apply p.patch

But I suspect there is a much easier/better way to do it.