revert "lei_store: use per-machine refname as git HEAD"

revert "lei_store: use per-machine refname as git HEAD"

In retrospect, per-machine HEADs was a bad idea because users
of removable storage would be thrown off when moving storage
between different machines.

This is only a partial revert, the Import::init_bare change to
support alternate head names still exists because we may use it
for other reasons.