bash completion: add basic support for git-reflog

bash completion: add basic support for git-reflog

"Promote" the reflog command out of plumbing, so that we now run
completion for it. After all, it's listed under porcelain (ancillary),
and we do run completion for those commands.

Add basic completion for the three subcommands - show, expire, delete.
Try completing refs for these too.

Helped-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>