bash: complete full refs

bash: complete full refs

Sometimes it's handy to complete full refs, e.g. the user has some
refs outside of refs/{heads,remotes,tags} or the user wants to
complete some git command's special refs (like 'git show
refs/bisect/bad').

To do that, we check whether the ref to be completed starts with
'refs/' or is 'refs' (to reduce the risk of matching 'refs-').  If it
does, then we offer full refs for completion; otherwise everything
works as usual.

This way the impact on the common case is fairly small (hopefully not
many users have branches or tags starting with 'refs'), and in the
special case the cost of typing out 'refs' is bearable.

While at it, also remove the unused 'cmd' variable from '__git_refs'.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>