When `hermes update` stashes local changes and the subsequent
`git stash apply` fails or leaves unmerged files, the conflict markers
(<<<<<<< etc.) were left in the working tree, making Hermes unrunnable
until manually cleaned up.
Now the update command runs `git reset --hard HEAD` to restore a clean
working tree before exiting, and also detects unmerged files even when
git stash apply reports success.
Closes#2348