How do I revert a merge commit that has already been pushed to remote?

git revert <commit_hash> alone won't work. Apparently, -m must be specified.

Solution

In git revert -m, the -m option specifies the parent number. This is needed because a merge commit has more than one parent, and Git does not know automatically which parent was the mainline, and which parent was the branch you want to un-merge.

When you view a merge commit in the output of git log, you will see its parents listed on the line that begins with Merge: (the second line in this case):

commit 8f937c683929b08379097828c8a04350b9b8e183
Merge: 8989ee0 7c6b236
Author: Ben James <ben@example.com>
Date:   Wed Aug 17 22:49:41 2011 +0100

Merge branch 'gh-pages'

Conflicts:
    README

In this situation, git revert 8f937c6 -m 1 will get you the tree as it was in 8989ee0, and git revert -m 2 will reinstate the tree as it was in 7c6b236.

To better understand what you're about to revert do git diff <parent_commit> <commit_to_revert>, in this case:

git diff 8989ee0 8f937c6

and

git diff 7c6b236 8f937c6

However, it's very important you realize that in doing so

"...declares that you will never want the tree changes brought in by the merge. As a result, later merges will only bring in tree changes introduced by commits that are not ancestors of the previously reverted merge. This may or may not be what you want. See the revert-a-faulty-merge How-To for more details." (git-merge man page).