completion: query only refs/heads/ in __git_refs_remotes()

completion: query only refs/heads/ in __git_refs_remotes()

__git_refs_remotes() is used to provide completion for refspecs to set
'remote.*.fetch' config variables for branches on the given remote.
So it's really only interested in refs under 'refs/heads/', but it
queries the remote for all its refs and then filters out all refs
outside of 'refs/heads/'.

Let 'git ls-remote' do the filtering.

Also remove the unused $cmd variable from __git_refs_remotes().

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>