git cat-file

git cat-file -p [MERGE_COMMIT_ID]

It shows the parent branches in order. The first one listed would be -m 1, the second -m 2.

git revert

git: [config | init | status ], git add, git rm, git commit, git push, git pull, .gitconfig, .gitignore, git (clone, fetch, pull), git tag, git log, git blame, Merge Request, LFS, git branch, git apply, git rebase, git diff, git diff-tree, git config --global, remote | submodule | request-pull, GIT_TRACE, GIT_CURL_VERBOSE, clean, main, git show-ref, git reset, git prune, git ls-remote origin, git checkout, git switch, pathspec, git credential-osxkeychain, git version, pre-commit, git --help, GitBook, refs/