I'm a Git user trying to use Mercurial.
Here's what happened: I did a
git reset --hard HEAD
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)