How do I git reset --hard HEAD on Mercurial?
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 andrestart the merge from scratch: "hg resolve file..." (or "-a" for all unresolved files)
This is close:
hg update --clean