Feeding "$ZERO_OID" to "git log --ignore-missing --stdin", and
running "git log --ignore-missing $ZERO_OID" fell back to start
digging from HEAD; it has been corrected to become a no-op, like
"git log --tags=no-tag-matches-this-pattern" does.

* jk/rev-input-given-fix:
  revision: set rev_input_given in handle_revision_arg()