completion: don't list 'HEAD' when trying refs completion outside of a repo

completion: don't list 'HEAD' when trying refs completion outside of a repo

When refs completion is attempted while not in a git repository, the
completion script offers 'HEAD' erroneously.

Check early in __git_refs() that there is either a repository or a
remote to work on, and return early if neither is given.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>