completion: make refs completion consistent for local and remote repos

completion: make refs completion consistent for local and remote repos

For a local repository the __git_refs() completion helper function
lists refs under 'refs/(tags|heads|remotes)/', plus some special refs
like HEAD and ORIG_HEAD.  For a remote repository, however, it lists
all refs.

Fix this inconsistency by specifying refs filter patterns for 'git
ls-remote' to only list refs under 'refs/(tags|heads|remotes)/'.

For now this makes it impossible to complete refs outside of
'refs/(tags|heads|remotes)/' in a remote repository, but a followup
patch will resurrect that.

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