Ref T13210. We currently "git reset --hard HEAD" during working copy leasing, mostly by convention/familiarity.
However, this command does not work in an empty repository, because there is no HEAD yet.
The command "git reset --hard" appears to have the same meaning and effect in all cases, except that it also works correctly in an empty repository.
The manual suggests that omitting HEAD should be the same as specifying HEAD, too:
The <tree-ish>/<commit> defaults to HEAD in all forms.