R1:6deb09efcdb4

When leasing a Working Copy in Drydock, just "git reset --hard" so empty repositories work

Summary:
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…