git revert -m
Undoes a whole merge by choosing which parent still counts.
Reverting an ordinary commit is trivial; a merge commit has two parents, and git needs to know which side of the union stays standing. -m 1 declares the first parent — usually the branch that received the merge — as the mainline, and the revert undoes everything the other side brought in.
The surgical warning, straight from the documentation: the revert undoes the data, not the ancestry. The reverted branch remains technically merged — to bring it back later, fixed, you must first revert the revert itself.
$ git revert -m 1 3e7a9c2
[main 8b1c4d5] Revert "Merge branch 'comentarios'"