There is a commit that just didn't work, so I want to abandon it without deleting it from history.
I have updated from an earlier revision and committed, thus creating a new head.
I don't have branches, I don't want branches, I just want to simply go on with the new head exactly as it is, nothing fancy, no merge, no worries, just go on forgetting the previous one.
I can't seem to find how to do that, and I'm starting to believe it can't be done. All I find is stuff about branches, or stuff about merging.
Update your repository to the head with the revision that you want to forget about, then use hg commit --close-branch to mark that (anonymous) branch as closed. Then update to the head of the branch that you do want, and continue working.
You can still see the closed branch if you use the -c option to hg heads, but it won't show up by default and hg merge will know not try to merge with the closed head.
You will need to use hg push --force the first time you push this closed head to another repository since you are actually create additional heads in the remote repository when you push. So tell Mercurial that this is okay with --force. People who pull the closed head wont be bothered by any warnings.