How do I git reset --hard HEAD on Mercurial?

I'm a Git user trying to use Mercurial.

Here's what happened: I did a hg backout on a changeset I wanted to revert. That created a new head, so hg instructed me to merge (back to "default", I assume). After the merge, it told me I still had to commit. Then I noticed something I did wrong when resolving a conflict in the merge, and decided I wanted to have everything as before the hg backout, that is, I want this uncommited merge to go away. On Git this uncommited stuff would be in the index and I'd just do a git reset --hard HEAD to wipe it out but, from what I've read, the index doesn't exist on Mercurial. So how do I back out from this?

If you've not yet commited, and it sounds like you haven't you can undo all the merge work with hg update --clean.

However, in newer mercurial's there's a handy command to re-merge a single file: hg resolve path/to/file.ext. From the hg help resolve:

> The available actions are: ... > > 4) discard your current attempt(s) at resolving conflicts and > restart > the merge from scratch: "hg resolve file..." (or "-a" for all > unresolved files)

This is close:

hg update --clean

From git, commands:

git reset --hard     # reset to last commit    
git clean -df        # delete untracked files

are equal to

hg update --clean    # reset to last commit  
hg purge             # delete untracked files

The Hg Manual’s GitConcepts page explains how to do many actions git users are familiar with in Mercurial.

Mercurial doesn’t have any built-in git reset --hard behavior. However, the strip extension provides a strip command which does. To use, first enable strip in your ~/.hgrc file::

[extensions]
strip =

Note: this extension is shipped in new in Mercurial 2.8. Prior versions provided the strip command in the mq extension.

Now you can run commands like hg strip or even hg help strip. To remove a changeset and all of its children, simply specify that changeset as an argument to hg strip. For example, to remove the last commit you just made (after you used commands which caused hg rollback to report that there is no longer any transaction to rollback), you can remove the tip revision. Each time you run this command, another revision will be removed. hg strip’s actions should be considered irreversible; unfamiliar users should make backups of their repositories before using.

$ hg strip tip

For example, with revsets syntax, I indicate that I want to remove any commits of mine which result in extra heads being shown when I run hg heads. If you specify a particular revision in the below expression other than tip, everything in the current branch that is not an ancestor of your chosen revision will be trimmed. This seems closest to the behavior I want when I issue the command git reset --hard HEAD.

$ hg strip "branch(tip) and not(ancestors(tip)::tip)"

What I expected to find as an alternative to git reset --hard in mercurial was:

hg strip --keep -r . # --keep optional
hg update --clean    # reset to last commit

And then if you need too:

hg purge