The right way to replicate git reset --soft HEAD^ (undo the current commit but keep changes in the working copy) is:
hg strip --keep -r .
-1 will only work if the commit you want to strip is the very last commit that entered the repository. . refers to the currently checked out commit, which is the closest equivalent Mercurial has to Git’s HEAD.
Note that if . has descendants, those will get stripped away too. If you’d like to keep the commit around, then once you have the commit ID, you can instead:
hg update .^
hg revert --all -r <commit id>
This will update to the commit’s parent and then replace the files in the working copy with the versions at that commit.