branch: restrict @-expansions when deleting

branch: restrict @-expansions when deleting

We use strbuf_branchname() to expand the branch name from
the command line, so you can delete the branch given by
@{-1}, for example.  However, we allow other nonsense like
"@", and we do not respect our "-r" flag (so we may end up
deleting an oddly-named local ref instead of a remote one).

We can fix this by passing the appropriate "allowed" flag to
strbuf_branchname().

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>